fastbootd: exporting CPU ABI info

CPU ABI info, e.g., arm64-v8a, can be useful to determine image
compatibility, prior to flash. Adding this info in fastbootd.

Bug: 74445765
Test: fastboot getvar cpu-abi # arm64-v8a
Change-Id: Ied494b646c551320295956b7890c0102fdb88382
4 files changed