sstate.bbclass: add SSTATEPOSTINSTFUNCS

When doing builds using sstate cache, there was no way to run
the equivalent of a pkg_postinst function. This is needed by
the SGML-related documentation recipes to properly update the
catalog files when new DTDs and stylesheets are installed.

SSTATEPOSTINSTFUNCS is a new variable you can set to function(s)
in your recipe to run after install is completed from sstate.

Thanks to Richard Purdie for suggesting this solution.

Signed-off-by: Scott Garman <scott.a.garman@intel.com>

meta/classes/sstate.bbclass

@@ -16,6 +16,8 @@ BB_HASHFILENAME = "${SSTATE_PKGNAME}"
 
 SSTATE_MANMACH ?= "${SSTATE_PKGARCH}"
 
+SSTATEPOSTINSTFUNCS ?= ""
+
 python () {
     if bb.data.inherits_class('native', d):
         bb.data.setVar('SSTATE_PKGARCH', bb.data.getVar('BUILD_ARCH', d), d)
@@ -129,6 +131,9 @@ def sstate_install(ss, d):
         f.write(di + "\n")
     f.close()
 
+    for postinst in (bb.data.getVar('SSTATEPOSTINSTFUNCS', d, True) or '').split():
+        bb.build.exec_func(postinst, d)
+
     for lock in locks:
         bb.utils.unlockfile(lock)