brcm63xx: call brcm63xx_detect explicitly

Now that we have converted all users of global variables to use
brcm63xx_board_name, we can savely convert the implicit call to detect
in brcm63xx.sh to a single, explicit call in preinit.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44840 3c298f89-4303-0410-b956-a3cf2f4a3e73