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.

Running wic in Debian 9 does not have this issue.

(From OE-Core rev: d6936301d7598b7a783beaae95109555faf6cc17)

Signed-off-by: Geoff Parker <geoffhp@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/engine.py

@@ -266,10 +266,15 @@ class Disk:
             out = exec_cmd("%s -sm %s unit B print" % (self.parted, self.imagepath))
             parttype = namedtuple("Part", "pnum start end size fstype")
             splitted = out.splitlines()
-            lsector_size, psector_size, self._ptable_format = splitted[1].split(":")[3:6]
+            # skip over possible errors in exec_cmd output
+            try:
+                idx =splitted.index("BYT;")
+            except ValueError:
+                raise WicError("Error getting partition information from %s" % (self.parted))
+            lsector_size, psector_size, self._ptable_format = splitted[idx + 1].split(":")[3:6]
             self._lsector_size = int(lsector_size)
             self._psector_size = int(psector_size)
-            for line in splitted[2:]:
+            for line in splitted[idx + 2:]:
                 pnum, start, end, size, fstype = line.split(':')[:5]
                 partition = parttype(int(pnum), int(start[:-1]), int(end[:-1]),
                                      int(size[:-1]), fstype)