MAINTAINERS: Add usr/ (initramfs generation) to KBUILD

Add scripts for generating initramfs to KBUILD, to prevent idling of
patches for usr/.
Signed-off-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

MAINTAINERS

@@ -11296,6 +11296,7 @@ F:	scripts/dummy-tools/
 F:	scripts/mk*
 F:	scripts/mod/
 F:	scripts/package/
+F:	usr/
 
 KERNEL HARDENING (not covered by other areas)
 M:	Kees Cook <keescook@chromium.org>