boards/hamilton: remove OFLAGS '-O binary'

[cladmi]

Contribution description

The 'binary' output format is now given by default to .bin files creation

This was changed before hamilton board integration but not updated in the PR.

Testing procedure

The -O binary flag is only once in the objcopy output and compilation gives the same .bin file.

objcopy

RIOT_CI_BUILD=1 BOARD=hamilton QUIET=0 make --no-print-directory -C examples/hello-world/ | grep objcopy
/opt/gcc-arm-none-eabi-7-2018-q2-update/bin/arm-none-eabi-objcopy --gap-fill 0xff -Obinary /data/riotbuild/riotbase/examples/hello-world/bin/hamilton/hello-world.elf /data/riotbuild/riotbase/examples/hello-world/bin/hamilton/hello-world.bin

In master it had two times -O binary.

BUILD_IN_DOCKER=1 RIOT_CI_BUILD=1 BOARD=hamilton make --no-print-directory -C examples/hello-world/ QUIET=0 | grep objcopy
/opt/gcc-arm-none-eabi-7-2018-q2-update/bin/arm-none-eabi-objcopy -O binary --gap-fill 0xff -Obinary /data/riotbuild/riotbase/examples/hello-world/bin/hamilton/hello-world.elf /data/riotbuild/riotbase/examples/hello-world/bin/hamilton/hello-world.bin

binary output

The binary is the same for both

sha256sum examples/hello-world/bin/hamilton/hello-world.bin
a8283b86d7e4a87bcaebde1447a3f419d968317044d2880aeb4b2b2a9d66fdec  examples/hello-world/bin/hamilton/hello-world.bin

Issues/PRs references

Part of the #8838 changes

[jcarrano]

Tested, works.

[cladmi]

Thank you for the review.