cmake: include/cmake.mk add CMAKE_BINARY_SUBDIR to allow out of source tree builds

Some packages need out of source tree building with cmake, for example
when building kernel modules.
See an example here:
https://sourceforge.net/p/accel-ppp/code/ci/master/tree/README

Signed-off-by: Luke McKee <hojuruku@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [cleanup, rework]

include/cmake.mk

@@ -6,7 +6,9 @@ ifneq ($(findstring c,$(OPENWRT_VERBOSE)),)
   MAKE_FLAGS+=VERBOSE=1
 endif
 
-CMAKE_SOURCE_DIR:=.
+CMAKE_BINARY_DIR = $(PKG_BUILD_DIR)$(if $(CMAKE_BINARY_SUBDIR),/$(CMAKE_BINARY_SUBDIR))
+CMAKE_SOURCE_DIR = $(PKG_BUILD_DIR)
+MAKE_PATH = $(firstword $(CMAKE_BINARY_SUBDIR) .)
 
 ifeq ($(CONFIG_EXTERNAL_TOOLCHAIN),)
   cmake_tool=$(TOOLCHAIN_DIR)/bin/$(1)
@@ -35,7 +37,8 @@ CMAKE_HOST_FIND_ROOT_PATH:=$(STAGING_DIR)/host;$(STAGING_DIR_HOST)
 CMAKE_SHARED_LDFLAGS:=-Wl,-Bsymbolic-functions
 
 define Build/Configure/Default
-	(cd $(PKG_BUILD_DIR); \
+	mkdir -p $(CMAKE_BINARY_DIR)
+	(cd $(CMAKE_BINARY_DIR); \
 		CFLAGS="$(TARGET_CFLAGS) $(EXTRA_CFLAGS)" \
 		CXXFLAGS="$(TARGET_CFLAGS) $(EXTRA_CFLAGS)" \
 		LDFLAGS="$(TARGET_LDFLAGS) $(EXTRA_LDFLAGS)" \

include/package-defaults.mk

@@ -136,7 +136,7 @@ MAKE_INSTALL_FLAGS = \
 	$(MAKE_FLAGS) \
 	DESTDIR="$(PKG_INSTALL_DIR)"
 
-MAKE_PATH = .
+MAKE_PATH ?= .
 
 define Build/Compile/Default
 	+$(MAKE_VARS) \