ti: fastboot: Move weak overrides to board files

Overriding fastboot_set_reboot_flag() in arch/arm/mach-omap2/boot-common.c
leaves it applying all boards that derive from this, not just the ones which
have support for Android bootloader flow. Move the weak function override to
the relevant board files.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>