license.bbclass: fix copying license directories failed

For each recipe, it populated license files to ${LICENSE_DIRECTORY}/${PN},
such as kernel's license dir was ${LICENSE_DIRECTORY}/kernel-3.10.17-yocto-standard;

In do_rootfs task, it copied license directories from ${LICENSE_DIRECTORY}/
${pkg}, and ${pkg} was listed in ${INSTALLED_PKGS};

We got ${INSTALLED_PKGS} by rpm query, such as the kernel were 'kernel-*',
but the kernel's PN was linux-yocto, so searching ${LICENSE_DIRECTORY}/
kernel-* failed.

Copied license directories from ${LICENSE_DIRECTORY}/${PN} fixed this
issue.

[YOCTO #5572]

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>