ramdisk_module and dtb_prebuild properties are optional

The two properties are now optional, so that we can split boot.img into
boot.img that doesn't have ramdisk and init_boot.img that has only the
ramdisk.

Bug: N/A
Test: m com.android.virt
Change-Id: I5dc8b140893dfbcf69a8b5c099d23b9216217b53