remove-pkg-postinst-ontarget: support other runtime packages

A recipe may generate different runtime packages, with names other than
PN. This commit allows removing the ontarget postinst script for those
other runtime package names. To do so, just define REMOVE_POSTINST_RPN
before including this class in the recipe.

The first user is in the following commit.

Signed-off-by: Javier Viguera <javier.viguera@digi.com>

meta-digi-arm/classes/remove-pkg-postinst-ontarget.bbclass

@@ -1,2 +1,7 @@
-pkg_postinst_ontarget:${PN}() {
+python __anonymous() {
+    RPN = d.getVar("REMOVE_POSTINST_RPN")
+    if RPN is None:
+        RPN = d.getVar("PN")
+    if RPN:
+        d.setVar('pkg_postinst_ontarget:%s' % RPN, "")
 }