xcrypt: work-around broken use of VARIANT with BUILDONLY

xcrypt: work-around broken use of VARIANT with BUILDONLY

Split the xcrypt package build into two Makefiles and a common part for
the version definition in order to work-around build problems when
combining VARIANT with BUILDONLY and scoped InstallDev.

This is done in order to skip build of libcrypt-compat in case we are
not building against glibc in order to prevent libcrypt.so shared
library being present in staging_dir and by that breaking multiple
packages which then will link against it.

Fixes: e3cf7088f1 ("libcrypt-compat: introduce package")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://github.com/openwrt/openwrt/pull/19353
Signed-off-by: Robert Marko <robimarko@gmail.com>