perl: fix PERL5LIB settings

The PERL5LIB settings in the perl wrapper script did not include the
"site_perl" or "vendor_perl" directories, which caused some errors.

See https://bugzilla.yoctoproject.org/show_bug.cgi?id=6890

Signed-off-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>