gradm: do not use 'exit 1' to postpone to first boot

Since `229f4e9 package.bbclass: add support for
pkg_postinst_ontarget()' applied in oe-core, use
pkg_postinst_ontarget to run postinst at first boot.

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-oe/recipes-support/gradm/gradm_3.1.bb

@@ -43,10 +43,6 @@ do_install() {
     rm -rf ${D}/dev
 }
 
-pkg_postinst_${PN}() {
-    # make sure running on the target
-    if [ x"$D" != "x" ]; then
-        exit 1
-    fi
+pkg_postinst_ontarget_${PN}() {
     /bin/mknod -m 0622 /dev/grsec c 1 13
 }