support/kconfig: Fix typo in README.buildroot.

Fix typo in README.buildroot (s/config/kconfig/).

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>