discover/yaboot-parser: Handle 'partition=' directive override

discover/yaboot-parser: Handle 'partition=' directive override

In a yaboot conf file, we may see a device= directive that actually
specifies a partition (eg, sda1) rather than the underlying block device
(sda). If we then encounter a partition= directive, we don't handle the
resolution of the partition correctly, as we simply append the
partition number to the device= string.

This change implements a smarter handling of the partition= directive,
where we strip away any partition information from the device=
parameter first.

Signed-off-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>