README: Remove outdated commands

Remove mentions of "selectcommunity" command and community file.
Start removing mentions of bsp which is not available.
Fix buildscript commands (there is no kernel config).

Signed-off-by: Andreas Düring <gh@duering-andreas.de>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>