Fix firstboot fs mount logic

The firstboot logic has an error which causes the filesystem to be
mounted readonly even though root_rw_mount=YES. This fixes the error to
ensure that the root filesystem is mounted rw as expected after the run
of the firstboot scripts.

Reviewed by:	imp
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D14226

etc/rc

@@ -141,10 +141,10 @@ if [ -e ${firstboot_sentinel} ]; then
 	if [ -e ${firstboot_sentinel}-reboot ]; then
 		chflags -R 0 ${firstboot_sentinel}-reboot
 		rm -rf ${firstboot_sentinel}-reboot
-    		checkyesno root_rw_mount && mount -ur /
+    		checkyesno root_rw_mount || mount -ur /
 		kill -INT 1
 	fi
-    	checkyesno root_rw_mount && mount -ur /
+    	checkyesno root_rw_mount || mount -ur /
 fi
 
 echo ''