unpack_bootimg: Change vendor ramdisk name index width to 2

In case there are more than 10 ramdisks, shell globbing like
"echo out/vendor_ramdisk*" can return the list in correct order.

For example: vendor_ramdisk00, vendor_ramdisk01 ... vendor_ramdisk10,
vendor_ramdisk11.

Bug: 180712476
Test: atest --host mkbootimg_test
Change-Id: I3f7f4725d79e0538a99ac0f01116c995dbd45a81