This commit adds support for the MacBook Air 4,2 from
Apple, circa 2011. This is a blobless board in coreboot,
when configured the way Libreboot does it, except for
the presence of neutered ME (via me_cleaner) and (per
Libreboot project policy), CPU microcode updates.

Intel ME image automatically extracted from Apple EFI
updates, via the same blobutil script used for fetching
Lenovo ME updates. This means that the user does not
have to extract it from a dump of the original firmware,
when flashing Libreboot.

Apple EFI updates are .dmg files, which are LZMA
compressed archives containing executables and files.
When the archive is de-compressed, more compressed
archives shall be found inside.

The file targeted (from Apple) contains Apple's EFI
firmware update program, that the user runs in MacOS,
but we don't care about running that. No, we are only
interested in Apple's ME update image, which we extract
as `me.bin` and then run through me_cleaner.

This has to be done through brute force, because the
location of that file is random, with the .dmg file.
The script systematically (and recursively) scans the
file, trying me_cleaner on files inside first, before
attempting extraction with 7z; if me_cleaner passes,
then we've found the me.bin file, or if 7z passes, we
scan the resulting directory; if neither pass, or the
directory created by 7z is finished being scanned, the
next file or directory is scanned.

When the script encounters a directory in each iteration
of the loop, that directory is scanned.

I wonder if a non-recursive method would be feasible in
shell scripts. This method shouldn't pose any problems,
though.