linux: Add Linux Kernel extensions menu

Add a kernel sub-menu (called "Linux Kernel Extensions"), which makes
possible to patch it or tweak the kernel build step.

 * All linux/linux-ext-*.mk files will be read by the make process.
 * The menu can be customized in "linux/Config.ext.in".

[Peter: small fixes]
Signed-off-by: Benoit Mauduit <benoit.mauduit@openwide.fr>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>