include: remove SDK specific target from toplevel.mk

Instead introduce a generic "make prepare" target which is executed if
include/prepare.mk exists.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 44329

include/toplevel.mk

@@ -136,13 +136,19 @@ kernel_menuconfig: prepare_kernel_conf
 kernel_nconfig: prepare_kernel_conf
 	$(_SINGLE)$(NO_TRACE_MAKE) -C target/linux nconfig
 
-tmp/.prereq-build: $(if $(SDK),.git/config) include/prereq-build.mk
+tmp/.prereq-build: include/prereq-build.mk
 	mkdir -p tmp
 	rm -f tmp/.host.mk
 	@$(_SINGLE)$(NO_TRACE_MAKE) -j1 -r -s -f $(TOPDIR)/include/prereq-build.mk prereq 2>/dev/null || { \
 		echo "Prerequisite check failed. Use FORCE=1 to override."; \
 		false; \
 	}
+  ifneq ($(realpath $(TOPDIR)/include/prepare.mk),)
+	@$(_SINGLE)$(NO_TRACE_MAKE) -j1 -r -s -f $(TOPDIR)/include/prepare.mk prepare 2>/dev/null || { \
+		echo "Preparation failed."; \
+		false; \
+	}
+  endif
 	touch $@
 
 printdb: FORCE