update_verifier: Support AVB.

When using AVB, PRODUCT_SUPPORTS_VERITY is not set so check for
BOARD_ENABLE_AVB as well. Also AVB sets up the root filesystem as
'vroot' so map that to 'system' since this is what is
expected. Managed to test at least that the code is at least compiled
in:

 $ fastboot --set-active=_a
 Setting current slot to 'a'...
 OKAY [  0.023s]
 finished. total time: 0.023s

 $ fastboot reboot
 rebooting...

 finished. total time: 0.050s

 $ adb wait-for-device

 $ adb logcat |grep update_verifier
 03-04 05:28:56.773   630   630 I /system/bin/update_verifier: Started with arg 1: nonencrypted
 03-04 05:28:56.776   630   630 I /system/bin/update_verifier: Booting slot 0: isSlotMarkedSuccessful=0
 03-04 05:28:56.776   630   630 W /system/bin/update_verifier: Failed to open /data/ota_package/care_map.txt: No such file or directory
 03-04 05:28:56.788   630   630 I /system/bin/update_verifier: Marked slot 0 as booted successfully.
 03-04 05:28:56.788   630   630 I /system/bin/update_verifier: Leaving update_verifier.

Bug: None
Test: Manually tested on device using AVB bootloader.
Change-Id: I13c0fe1cc5d0f397e36f5e62fcc05c8dfee5fd85