trustfence: fix path creation and calling on copy_public_key()

In the context of the class, we must use ${IMAGE_ROOTFS} instead of ${D}.
Change the calling of the function to POSTPROCESS (after the rootfs has
been created) instead of POSTINSTALL (after the packages have been
installed).

Signed-off-by: Hector Palacios <hector.palacios@digi.com>

meta-digi-dey/classes/trustfence.bbclass

@@ -97,10 +97,10 @@ copy_public_key() {
 		exit 1
 	fi
 	# Copy the public key to the rootfs
-	install -d ${D}${sysconfdir}/ssl/certs
+	install -d ${IMAGE_ROOTFS}${sysconfdir}/ssl/certs
 	cp -f "${PUBLIC_KEY}" "${IMAGE_ROOTFS}${sysconfdir}/ssl/certs/key.pub"
 }
-ROOTFS_POSTINSTALL_COMMAND:append = " copy_public_key;"
+ROOTFS_POSTPROCESS_COMMAND:append = " copy_public_key;"
 
 python () {
     import binascii