tools: Use PKG_CONFIG_FILE instead of PKG_CONFIG variable

tools: Use PKG_CONFIG_FILE instead of PKG_CONFIG variable

Replace PKG_CONFIG variable name with PKG_CONFIG_FILE for the name of
the pkg-config file.
This is preventing a conflict in some build systems where PKG_CONFIG
actually contains the path to the pkg-config executable to use, as the
default assignment in libs.mk is using a weak assignment (?=).

This problem has been found when trying to build the latest version of
Xen tools using buildroot.

Fixes: d400dc5729e4 ("tools: tweak tools/libs/libs.mk for being able to support libxenctrl")
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
master commit: b97e2fe7b9e1f4706693552697239ac2b71efee4
master date: 2023-02-24 17:44:29 +0000