package/pkg-generic: ensure absolute paths in config-script fixups

package/pkg-generic: ensure absolute paths in config-script fixups

In case a config script is called from a relative path, the $(dirname
$0) would return a relative path too.

Those paths are usually parts of includes or libraries search
directories, and the packagfes buildsystems may chdir() anywhere, and
thus the relative path will no longer be valid. For example:

  $ ./host/powerpc-buildroot-linux-uclibc/sysroot/usr/bin/net-snmp-config --cflags
  [...] -I./host/powerpc-buildroot-linux-uclibc/sysroot/usr/bin/../../.././bin/../powerpc-buildroot-linux-uclibc/sysroot/usr/include/libnl3 [...]

Canonicalise the path to be sure we use absolute paths.

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