px5g: generate unique serial numbers

Generate a random serial from /dev/urandom when creating selfsigned certs.
Fixes "sec_error_reused_issuer_and_serial" with Firefox.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43168 3c298f89-4303-0410-b956-a3cf2f4a3e73