img_from_target_files: Create md5 hashes for each file

Generate an md5sum-fastboot.md5 file that contains an md5 hash of each
file in the fastboot zip and store the file in the zip.

Change-Id: I19c2ac5a84fb21e965953d1dad0ebcbfd0b78d47
Merge branch 'cm-11.0' of github.com:CyanogenMod/android_build into cm-11.0