swig: add native wrapper for SWIGLIB

Native swig will read data from hard-coded SWIGLIB or the same
environment variable.

While using sstate, the hard-coded SWIGLIB will point to the project
that create original sstates. This would cause build issues, so add
a wrapper to set the environment variable SWIGLIB to a relative path
on current sysroot.

Signed-off-by: Xin Ouyang <Xin.Ouyang@windriver.com>
Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>