make_dev_firmware: provide more usage help for backup files

Provide more clear instruction on how to use the backup files,
and to try more effort to store backup files

BUG=none
TEST=emerge-x86-generic vboot-reference; executed make_dev_firmware and got correct message

Change-Id: I2062f45dd3019d0e56adc18bdd1861991aafe5ed

Review URL: http://codereview.chromium.org/3785014
1 file changed