wic: make engine.py:get_partitions() resilient to parted/dmidecode stderr output
Running wic commands on Debian 10 systems fail in
scripts/lib/wic/engine.py:get_partitions() due to new stderr output captured
when trying to parse the output from /sbin/parted as a non-root user.
The parted command calls the dmidecode utility, which produces this error
as a non-root user:
/sys/firmware/dmi/tables/smbios_entry_point: Permission denied
/dev/mem: Permission denied
scripts/lib/wic/engine.py:get_partitions() calls misc.py:exec_cmd(),
a subprocess wrapper which returns a combined stderr and sdtdout.
These messages to stderr confuse the partition table parser in
get_partitions().
This patch has the partition table parser ignore lines before the expected
"BYT;" header string.
