build: add support for host building in a subdirectory

Add HOST_MAKE_PATH and use it in order to execute Make
in a subdirectory of the build directory
and in a similar way that MAKE_PATH is used for target building.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/15991
Signed-off-by: Robert Marko <robimarko@gmail.com>

include/host-build.mk

@@ -111,9 +111,11 @@ define Host/Configure
   $(call Host/Configure/Default)
 endef
 
+HOST_MAKE_PATH ?= .
+
 define Host/Compile/Default
 	+$(HOST_MAKE_VARS) \
-	$(MAKE) $(HOST_JOBS) -C $(HOST_BUILD_DIR) \
+	$(MAKE) $(HOST_JOBS) -C $(HOST_BUILD_DIR)/$(HOST_MAKE_PATH) \
 		$(HOST_MAKE_FLAGS) \
 		$(1)
 endef