cleanup: Allow build for only one variant and other misc

-Error handling if no vendor blobs are found.

-Notes about permissions for future bring-ups.
vendor files: Detect variant by baseband version.

-Instead of requiring variant specific recoverys, we mount
modemfs (/dev/block/mmcblk0p12) and check the baseband version
in PERM/PROT/NV.DAT
releasetools: Allow building for only 1 variant.

-Now the build environment can build all three or, if you only
want to build one, it can do that too.

-Vendor files for at least one variant must be present...