BB: dropbear: restore performance by disabling mips16

Disable MIPS16 to prevent it negatively affecting performance.
Observed was a increase of connection delay from ~6 to ~11 seconds
and a reduction of scp speed from 1.1MB/s to 710kB/s on brcm63xx.

Fixes #15209.

Backport of r42250.

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

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42255 3c298f89-4303-0410-b956-a3cf2f4a3e73