include/cmake.mk: add support for overriding the host install prefix

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/cmake.mk

@@ -52,6 +52,7 @@ CMAKE_RANLIB:=$(call cmake_tool,$(TARGET_RANLIB))
 CMAKE_FIND_ROOT_PATH:=$(STAGING_DIR)/usr;$(TOOLCHAIN_DIR)$(if $(CONFIG_EXTERNAL_TOOLCHAIN),;$(CONFIG_TOOLCHAIN_ROOT))
 CMAKE_HOST_FIND_ROOT_PATH:=$(STAGING_DIR)/host;$(STAGING_DIR_HOSTPKG);$(STAGING_DIR_HOST)
 CMAKE_SHARED_LDFLAGS:=-Wl,-Bsymbolic-functions
+CMAKE_HOST_INSTALL_PREFIX = $(HOST_BUILD_PREFIX)
 
 ifeq ($(HOST_USE_NINJA),1)
   CMAKE_HOST_OPTIONS += -DCMAKE_GENERATOR="Ninja"
@@ -157,7 +158,7 @@ define Host/Configure/Default
 			-DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY \
 			-DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY \
 			-DCMAKE_STRIP=: \
-			-DCMAKE_INSTALL_PREFIX=$(HOST_BUILD_PREFIX) \
+			-DCMAKE_INSTALL_PREFIX=$(CMAKE_HOST_INSTALL_PREFIX) \
 			-DCMAKE_PREFIX_PATH=$(HOST_BUILD_PREFIX) \
 			-DCMAKE_SKIP_RPATH=TRUE  \
 			-DCMAKE_INSTALL_LIBDIR=lib \