classes/update-alternatives: Skip alternatives when disabled

classes/update-alternatives: Skip alternatives when disabled

Skips the update alternative steps for recipes that shouldn't have them
enabled.

Fixes errors like:

 nativesdk-bzip2-1.0.6-r5 do_package: bzip2: alternative target
 (/opt/poky/2.5+snapshot/sysroots/i686-pokysdk-mingw32/usr/bin/bunzip2
 or
 /opt/poky/2.5+snapshot/sysroots/i686-pokysdk-mingw32/usr/bin/bunzip2.bzip2)
 does not exist, skipping...

When building mingw SDKs

[YOCTO #12962]

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>