cmd: avb: Fix requested partitions list

The requested_partitions[] array should contain only boot partitions.
Usually it's only 'boot' partition, as can be seen in [1]. Also, seems
like the requested_partitions[] are only used when there is no 'vbmeta'
partition [2], which is not a regular use-case.

Make requested_partitions[] contain only 'boot' partition as it was
supposed to be, and also make that array to be a local in
do_avb_verify_part() function, as nobody else needs that.

[1] https://android.googlesource.com/platform/external/avb/+/5fbb42a189aa/test/avb_slot_verify_unittest.cc#108
[2] https://android.googlesource.com/platform/external/avb/+/5fbb42a189aa/libavb/avb_slot_verify.c#1461



Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>