Genex: Fix evaluation of MAP_IMPORTED_CONFIG_<CONFIG>

Commit 10a069b5 (Genex: Fix $<CONFIG> with IMPORTED targets and
multiple locations., 2013-07-15) changed the logic here to include
handling of the MAP_IMPORTED_CONFIG_<CONFIG> target property, but
it was buggy in several ways.

Uppercase the configs in all cases, and compare the mapped configs
with the parameter to the CONFIG genex, instead of with the key of
the mapping.
Merge topic 'fix-MAP_IMPORTED_CONFIG-evaluation'

15d98a4 Genex: Fix evaluation of MAP_IMPORTED_CONFIG_<CONFIG>