part: fix "part list ... -bootable varname" to use hex

Unfortunately U-Boot assumes that almost all numbers are in hex, including
partition numbers passed to e.g. "load". So, the command "part list mmc 0
-bootable devplist" should use hex when writing partition numbers into
$devplist, so they'll be correctly interpreted.

Change-Id: I9a70b19749643876baadb45efbc3decaef8bfee2
Fixes: 0798d6fd ("part: Add support for list filtering on bootable partitions")
Cc: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>