mkbootimg: Disallow special vendor ramdisk names

Disallow "default" as a vendor ramdisk name, as it has special meaning
for fastboot. For example,

  fastboot flash vendor_boot:default ramdisk.img

Bug: 173654501
Test: Presubmit
Change-Id: I709f19d126c7826806d8523aba98e4356925fc91
1 file changed