mkbootimg changes for bootconfig

These are changes to vendor boot image v4.
Append the vendor bootconfig file to the end of the vendor boot image
and add the size of the file to the vendor boot header.
unpack_bootimg finds the size in the header, and unpacks the bootconfig
file that was used to create the image.

Test: Create the new bootconfig file and verify that is in
vendor_boot.img using (1) unpack_bootimg and (2) cuttleifish bootloader
Test: atest --host mkbootimg_test
Bug: 173815685

Change-Id: I1816c19555f7263f7f7a33a57050a65324df5717