Update pre_bootloader_serial.py for RPi 5 architecture

[kirkpsmith]

Closes #136

[jalinei]

CI build test failed. I think it is because of the comment after the backslash.
Maybe we can enhance the syntax by adding parenthesis like below ?

mcumgr_executable = (
    "mcumgr"     if platform.system() == 'Linux' and platform.machine() == 'x86_64' else
    "mcumgr-rpi" if platform.system() == 'Linux' and platform.machine() == 'armv7l' else
    "mcumgr-rpi" if platform.system() == 'Linux' and platform.machine() == 'aarch64' else
    "mcumgr-mac" if platform.system() == 'Darwin' else
    "mcumgr.exe" if platform.system() == 'Windows' else
    None
)

[jalinei]

Just merged @kirkpsmith, thanks for the contribution 😃