AVB: support BOARD_BUILD_SYSTEM_ROOT_IMAGE

Passing --setup_rootfs_from_kernel to avbtool when
BOARD_BUILD_SYSTEM_ROOT_IMAGE is true

Bug: 33590159
Test: 'make' sailfish with BOARD_AVB_ENABLE := true
Test: 'make dist' with BOARD_AVB_ENABLE := true
Change-Id: Ieb58dd9ae6be1eceb90a33c739b85cff5cbc6e0a
2 files changed