Complain loudly if BR2_GLOBAL_PATCH_DIR does not exit

If the BR2_GLOBAL_PATCH_DIR is misspelled in the .config or if somebody
renamed the existing directory with patches, buildroot happily builds
everything but without the patches. As this can lead to surprising
results, it is better to fail and give a user a message that
BR2_GLOBAL_PATCH_DIR is wrongly configured.

Signed-off-by: Michal Sojka <sojka@merica.cz>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>