uboot-mxs: fix build on hosts lacking pkg-config

The uboot-mvebu package incorrectly used the host pkg-config for the tool
build parts, which broke the build on systems lacking pkg-config and only
worked by accident on those that have it installed.

Export the host-build specific environment variables for the uboot build
to redirect pkg-config invocations to our staged host build pkg-config in
buildroot.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>