Improve config host_explore when PKG_CONFIG_* are set
Improve config host_explore when PKG_CONFIG_* are set

Also allow PKG_CONFIG_SYSROOT_DIR to handle %MCONFIGDIR%, as this should
be a usual way to handle path prefix change, rather than utilizing
PKG_CONFIG_FLAGS.

Change-Id: Ib015e427df63777cccffcc35a24e22eabad2bcd9
Signed-off-by: Michal Przeplata <e7e66087ea2b6f4d3eb01c4545d71a74d74ace14@arm.com>

ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build