meson: set the sysroot in the cross files

meson: set the sysroot in the cross files

When cross compiling we have a sysroot, so by telling Meson where it is
explicitly it can do the right thing.

Currently this means it just sets PKG_CONFIG_SYSROOT_DIR to the value
that we already set it to, but in the future this can be used to stop
Meson looking directly in /usr when it shouldn't.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>