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
(cherry picked from commit 62776cfdb5865fdba5d6fb8dde5d57c609a47085)
releasetools: Properly handle custom values for tools

Rather than writing these values to their own files, store them
in the target_files META/misc_info.txt. This way, we don't need to
download and parse through multiple files when using signing/release
tools.

Change-Id: I6f845fc055e4ef2ed4adf9628cca505e99b2a9fa
(cherry picked from commit d8b439aeb823870d4c4160f491cd26fe5f5de030)