classes/buildhistory: fix reading of package-specific values from pkgdata

When writing out variable values to pkgdata, if the value has been set
in the datastore with an override for the package, we use the package
name override in the pkgdata key as well; however the recently added
code to read pkgdata in buildhistory.bbclass was just using the override
where we normally expect to have it. However, if a recipe overrides one
of the values that is normally set for the recipe on a per-package basis
(e.g. the external-sourcery-toolchain recipe sets PKGV this way) then
this led to KeyErrors. Re-write the pkgdata loading code to always strip
off the package name override if it is present.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>