Adding BOOT_OS_VERSION for the AVB prop in a boot.img

Devices using a prebuilt boot.img, e.g., a GKI, will re-sign
the boot.img to append AVB footers. Currently the value of
"com.android.build.boot.os_version" in the AVB footer is set
to the Android platform version. However, the GKI image might
be from another release branch, e.g., using a Android12-5.10
GKI in a Android 13 codebase.

Adding BOOT_OS_VERSION to allow specify the value separately.

Bug: 211364958
Test: make, then `avbtool info_image --image $OUT/boot.img`.
Change-Id: I335cc7b6c5a455d566f364df6a7dae79facc7a87