nativesdk.bbclass: set PKGDATA_DIR explicitly

nativesdk.bbclass: set PKGDATA_DIR explicitly

For now, if we enable 'multilib' and execute `bitbake uninative-tarball
-c populate_sdk', the command would fail, complaining nativesdk-glibc not
found in package feed.

The problem is that PKGDATA_DIR is not set correctly for nativesdk packages
when multilib is enabled.

For now, we have:
meta/conf/bitbake.conf:PKGDATA_DIR = "${STAGING_DIR_HOST}/pkgdata"

The above is overriden by the following is multilib is enabled.
meta/conf/multilib.conf:PKGDATA_DIR = "${STAGING_DIR}/${MACHINE}/pkgdata"

This is correct for normal packages. However, for nativesdk packages, this
is not correctly. This is because nativesdk.bbclass set STAGING_DIR_HOST as
follows.

meta/classes/nativesdk.bbclass:STAGING_DIR_HOST = "${STAGING_DIR}/${MULTIMACH_HOST_SYS}"

The above overrides the value in multilib.conf.

That's why we need to also set PKGDATA_DIR explicitly in nativesdk.bbclass.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>