package/skeleton: parameterise the symlinks-or-mkdir macro

currently, we only make the merged symlinks or non-merged mkdirs in the
target directory. This is fine, as long as one does not run gdb.

However, as soon as one wants to run gdbserver on the target,
gdb will only search for libraries in /lib in the sysroot.

In preparation for a patch to fix that, make the symlink-or-mkdir macro
a function that takes as parameter the base directory where to create
symlinks or mkdir in.

Move the function declarations out of the custom/non-custom skeleton
conditional block, so they are always available.

Reported-by: Pieterjan Camerlynck <pieterjan.camerlynck@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Pieterjan Camerlynck <pieterjan.camerlynck@gmail.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>