kernel: do not try to probe builtin modules on empty kmod package install

Builtin modules are always present, and trying to load them will cause
modprobe to spew errors when installing the empty kmod packages.

Fix this by never generating any postinst module install instructions
for builtin modules.

Fixes #842.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>

include/kernel.mk

@@ -136,6 +136,7 @@ define ModuleAutoLoad
 		local mod; \
 		shift 2; \
 		for mod in $$$$$$$$mods; do \
+			[ -e $(2)/$(MODULES_SUBDIR)/$$$$$$$$mod.ko ] || continue; \
 			mkdir -p $(2)/etc/modules.d; \
 			echo "$$$$$$$$mod" >> $(2)/etc/modules.d/$(1); \
 		done; \
@@ -154,6 +155,7 @@ define ModuleAutoLoad
 		local mod; \
 		shift 3; \
 		for mod in $$$$$$$$mods; do \
+			[ -e $(2)/$(MODULES_SUBDIR)/$$$$$$$$mod.ko ] || continue; \
 			mkdir -p $(2)/etc/modules.d; \
 			echo "$$$$$$$$mod" >> $(2)/etc/modules.d/$$$$$$$$priority-$(1); \
 		done; \