ramips: enhance user-space board detection

Similar to ar71xx, detect only once and cache the strings in
/tmp/sysinfo/model and /tmp/sysinfo/boardname.

Signed-off-by: Daniel Golle <dgolle@allnet.de>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 35850