flasher: only update the boot files if /boot/uboot/initrd.img is missing (function ignored in init=flasher version)

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
Prevent init-eMMC flasher from exiting
Type "beagleboard.org" to drop to a bash shell for debugging following
write_failure or successful completion.

Signed-off-by: Charles Steinkuehler <charles@steinkuehler.net>