boards/samd21-bootloader*: small cleanup

[aabadie]

Contribution description

This PR is a follow-up of #13664 and #13791 for samd21 boards that are using the arduino bootloader:

• exclude the bootloader module also when slipdev_stdio is used
• exclude the reset.c build when usb-board-reset is loaded instead of samd21-arduino-bootloader. This is more consistent I think. I skipped that initially when reviewing boards: set samd21-arduino-bootloader as DEFAULT_MODULE #13664

Testing procedure

• The code for managing the arduino bootloader is correctly built by default:

feather-m0

$ make BOARD=feather-m0 -C examples/hello-world
make: Entering directory '/work/riot/RIOT/examples/hello-world'
Building application "hello-world" for "feather-m0" with MCU "samd21".

"make" -C /work/riot/RIOT/boards/feather-m0
"make" -C /work/riot/RIOT/boards/common/samd21-arduino-bootloader
"make" -C /work/riot/RIOT/core
"make" -C /work/riot/RIOT/cpu/samd21
"make" -C /work/riot/RIOT/cpu/cortexm_common
"make" -C /work/riot/RIOT/cpu/cortexm_common/periph
"make" -C /work/riot/RIOT/cpu/sam0_common
"make" -C /work/riot/RIOT/cpu/sam0_common/periph
"make" -C /work/riot/RIOT/cpu/samd21/periph
"make" -C /work/riot/RIOT/drivers
"make" -C /work/riot/RIOT/drivers/periph_common
"make" -C /work/riot/RIOT/sys
"make" -C /work/riot/RIOT/sys/auto_init
"make" -C /work/riot/RIOT/sys/auto_init/usb
"make" -C /work/riot/RIOT/sys/event
"make" -C /work/riot/RIOT/sys/isrpipe
"make" -C /work/riot/RIOT/sys/newlib_syscalls_default
"make" -C /work/riot/RIOT/sys/pm_layered
"make" -C /work/riot/RIOT/sys/tsrb
"make" -C /work/riot/RIOT/sys/usb/usbus
"make" -C /work/riot/RIOT/sys/usb/usbus/cdc/acm
"make" -C /work/riot/RIOT/sys/usb_board_reset
   text	   data	    bss	    dec	    hex	filename
  15316	    132	   5868	  21316	   5344	/work/riot/RIOT/examples/hello-world/bin/feather-m0/hello-world.elf

• The bootloader is correctly removed from build when another stdio module is used, for example stdio_null:

feather-m0

$ USEMODULE=stdio_null make BOARD=feather-m0 -C examples/hello-world
make: Entering directory '/work/riot/RIOT/examples/hello-world'
Building application "hello-world" for "feather-m0" with MCU "samd21".

"make" -C /work/riot/RIOT/boards/feather-m0
"make" -C /work/riot/RIOT/core
"make" -C /work/riot/RIOT/cpu/samd21
"make" -C /work/riot/RIOT/cpu/cortexm_common
"make" -C /work/riot/RIOT/cpu/cortexm_common/periph
"make" -C /work/riot/RIOT/cpu/sam0_common
"make" -C /work/riot/RIOT/cpu/sam0_common/periph
"make" -C /work/riot/RIOT/cpu/samd21/periph
"make" -C /work/riot/RIOT/drivers
"make" -C /work/riot/RIOT/drivers/periph_common
"make" -C /work/riot/RIOT/sys
"make" -C /work/riot/RIOT/sys/auto_init
"make" -C /work/riot/RIOT/sys/newlib_syscalls_default
"make" -C /work/riot/RIOT/sys/pm_layered
"make" -C /work/riot/RIOT/sys/stdio_null
   text	   data	    bss	    dec	    hex	filename
   7748	    112	   2544	  10404	   28a4	/work/riot/RIOT/examples/hello-world/bin/feather-m0/hello-world.elf

• Same when the slipdev_stdio module is used:

feather-m0

$ USEMODULE=slipdev_stdio make BOARD=feather-m0 -C examples/hello-world
make: Entering directory '/work/riot/RIOT/examples/hello-world'
Building application "hello-world" for "feather-m0" with MCU "samd21".

"make" -C /work/riot/RIOT/boards/feather-m0
"make" -C /work/riot/RIOT/core
"make" -C /work/riot/RIOT/cpu/samd21
"make" -C /work/riot/RIOT/cpu/cortexm_common
"make" -C /work/riot/RIOT/cpu/cortexm_common/periph
"make" -C /work/riot/RIOT/cpu/sam0_common
"make" -C /work/riot/RIOT/cpu/sam0_common/periph
"make" -C /work/riot/RIOT/cpu/samd21/periph
"make" -C /work/riot/RIOT/drivers
"make" -C /work/riot/RIOT/drivers/periph_common
"make" -C /work/riot/RIOT/drivers/slipdev
"make" -C /work/riot/RIOT/sys
"make" -C /work/riot/RIOT/sys/auto_init
"make" -C /work/riot/RIOT/sys/isrpipe
"make" -C /work/riot/RIOT/sys/newlib_syscalls_default
"make" -C /work/riot/RIOT/sys/pm_layered
"make" -C /work/riot/RIOT/sys/tsrb
   text	   data	    bss	    dec	    hex	filename
   9184	    132	   2620	  11936	   2ea0	/work/riot/RIOT/examples/hello-world/bin/feather-m0/hello-world.elf

Issues/PRs references

Follow-up of #13664 and #13791

[fjmolinas]

ACK, thanks for the cleanup.