Add bootimg module type

The new module type is to build the boot partition images. Boot
partition image consists of kernel, ramdisk, dtb, etc. which are
processed by the mkbootimg tool. There are two flavors of boot
partition: boot and vendor_boot. The choice is configured via the
'vendor_boot' property. Currently, only `vendor_boot: true` case is
supported.

But: 178978059
Test: m cf_bootimg-5.10
Change-Id: Iee689d9d666e8e6e5af7db02a76dd39d1e4d0e3a