Revert "fastboot: Add default fastboot_set_reboot_flag implementation"

This reverts commit 0ebf9842.

Current generic implementation of fastboot_set_reboot_flag is somewhat
messy and requires some additional configuration option to be enabled
besides CMD_BCB, so it reverts that implementtion in order to bring a
new cleaner one.

Next commit introduces new generic implementation of
fastboot_set_reboot_flag.

Signed-off-by: Roman Kovalivskyi <roman.kovalivskyi@globallogic.com>