unpack_bootimg: saves mkbootimg arguments to a .json file

This makes it easier for us to repack a boot.img/vendor_boot.img
later, where we can load the .json file, convert the config to
mkbootimg arguments.

Bug: 176789512
Test: m unpack_bootimg
Test: unpack_bootimg --boot_img $OUT/boot.img --out boot
Test: unpack_bootimg --boot_img $OUT/vendor_boot.img --out vendor_boot
Test: atest mkbootimg_test --host

Change-Id: Ica9aa6916dbf3669e9378ce375bf0695c3428fcc