buildscript: Remove prepare from buildall command

The builds for all BSPs have been merged into a single OpenWRT buildroot.
A prepare between builds cleans everything that has already been built,
including host tools. Building for multiple targets therefore takes quite
a bit longer than it has to.

This removes the prepare command between builds in the buildall command.
It now behaves exactly like multiple conscutive "selectbsp, build" commands,
which speeds up the build for multiple devices a lot.

This now means, that prepare has to be executed before buildall can be used,
just like with the 'build' command.

Signed-off-by: Fabian Bläse <fabian@blaese.de>

buildscript

@@ -382,7 +382,6 @@ setBSP()
 buildall() {
     for bsp in $(/bin/ls bsp/*.bsp); do
         setBSP "$bsp"
-        prepare
         build "$1"
     done
 }