package.bbclass: add support for pkg_postinst_ontarget()

package.bbclass: add support for pkg_postinst_ontarget()

This function is a convenient and more readable shortcut for situations
when the postinst code always needs to run on target. All commands that
cannot be executed during cross-install and can only be run on target
should go into this function. They will only be executed on first boot
(if package was cross-installed) or immediately during package installation
on target.

Plain pkg_postinst() works as before: it is run during cross-install time,
it can contain a request to defer to first boot, and it is also run
during package installation on target.

Also fix the oeqa test for this functionality to use the new function
where appropriate.

(From OE-Core rev: 229f4e975fb6957f44b5c56735fd6d58564098d7)

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>