Add a ramdisk-install.d hook path.

We need to be able to do install.d like things for ramdisks
themselves, but install.d runs outside the ramdisk context - and its
likely to break peoples brains if we mangle the two together - so this
adds a new hook point, ramdisk-install, specifically for installing
things into the ramdisk.

Change-Id: I37d1660309cda6e28bd0b316b08f61db4e080613
Merge "Add a ramdisk-install.d hook path."