Refactor dexpreopt_bootjars.go to prepare for multiple boot images

Refactor dexpreopt_bootjars.go so that buildBootImage can be called
with a bootImageConfig to create rules to build the boot image in
preparation for building multiple boot images.

Test: m checkbuild
Change-Id: I7ba40a0988c8acbd6c1ee4e36f4bbb34fb4e2d24