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 pull request #32 from CyanogenMod/cm-11.0

img_from_target_files: Create md5 hashes for each file