skeleton: move LIB_SYMLINK definition from Makefile

Commit 7a6b83a2 introduced the skeleton
package, which took over the lib32/lib64 -> lib symlink creation from the
main Makefile.
However, the definition of the LIB_SYMLINK variable did not move along, for
no real reason.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>