pkgconf: use relative path to STAGING_DIR instead of absolute path

The pkg-config wrapper script is currently generated with absolute
paths to $(STAGING_DIR). However, this will not work properly with
per-package SDK, and each package will be built with a different
STAGING_DIR value.

In order to fix this, we adjust how the pkg-config wrapper script is
generated, so that it uses a relative path to itself: the sysroot (i.e
STAGING_DIR) is always located in $(path of
pkg-config)/../$(STAGING_SUBDIR).

This change is independent from the per-package SDK work, and could be
applied independently from it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>