oe-pkgdata-util: Make read-value handle override variables

oe-pkgdata-util: Make read-value handle override variables

Some variables in pkgdata files have a package-name override.  When
the bare variable can not be found, try with the override-variant.

PKGSIZE is one such variable, and already had special code to handle this.

Test included.

Signed-off-by: Ola x Nilsson <ola.x.nilsson@axis.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>