package: check PKG_ variables before executing ontarget postinst

package: check PKG_ variables before executing ontarget postinst

If a package uses PKG_ variables to map package names to version specific
variants, on target postinstall functionality will be broken.

i.e. something like the following casuses rootfs assembly errors:

  d.setVar('pkg_postinst_ontarget_linux-source', 'cd /usr/src/; ln -sf %s linux-source' % source_pkg)

This breakage is due to the fact that the original package name (as specified by
the PACKAGES variable) is logged by the intercept scripts, but the mapped /
specific version is actually installed to the rootfs (and hence logged by the
package manager).

When the runtime listing of on-target scripts is performed, we get a package
manager error due to a missing package, since it checks the generic version
logged by the intercept scripts.

We can fix this by ensuring that the PKG_ variable mapped package name
is logged by the intercept phase, and hence the package manager can locate
and execute the on target postinst script.

This variable check is consistent with other places in the code, and has
no impact if PKG_ variables are not used.

Signed-off-by: Bruce Ashfield <bruce.ashfield@xilinx.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>