Update serial.c

prevent user to reboot if armed, due to cli and save or reboot to bootlooader.

https://github.com/varesa/baseflight/commit/4f3ab5b9913cc4b5bfcb068d1ce7b9dee746277a
Merge pull request #64 from treymarc/patch-2

prevent user to reboot if armed
Merge remote-tracking branch 'multiwii/master'

Conflicts:
	src/serial.c