fastboot: Rename fb_set_reboot_flag to fastboot_set_reboot_flag

Rename fb_set_reboot_flag to fastboot_set_reboot_flag so it matches
all other fastboot code in the global name space. Fix the guards around
them so that they're dependent on FASTBOOT, not just USB_FUNCTION_FASTBOOT.

Move the weak implementation of fastboot_set_reboot_flag to fb_common.c
so we can call it from non-USB fastboot code.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>