Config.in files: unification of comments about dependency on Linux kernel

This patch lines up the comments of packages that need a Linux kernel to be
built by buildroot, to the format:
    foo needs a Linux kernel to be built

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>