Add prebuilt bootloader var and add bootloader to img zip

Needed to enable a prebuilt bootloader to make it to the dist and img
directories.

Added a flag to ensure only devices that request will have the update
package populated with a bootloader

Bug: 164917252
Test: local build and Treehugger build
Change-Id: Id67b7bddda14b51a523f794976591c6de04d71ca