Revert "include: use default CCACHE_DIR."

This reverts commit c1f879cd.
This will be superseded by OpenWrt 21.02 config option CCACHE_DIR.

include/host-build.mk

@@ -132,9 +132,7 @@ define Host/Exports/Default
   $(1) : export STAGING_PREFIX=$$(HOST_BUILD_PREFIX)
   $(1) : export PKG_CONFIG_PATH=$$(STAGING_DIR_HOST)/lib/pkgconfig:$$(HOST_BUILD_PREFIX)/lib/pkgconfig
   $(1) : export PKG_CONFIG_LIBDIR=$$(HOST_BUILD_PREFIX)/lib/pkgconfig
-# Do not set CCACHE_dir. We want to use the home directory of the user.
-# otherwise nigtly and new builds will not see a speedup.
-#  $(1) : export CCACHE_DIR:=$(STAGING_DIR_HOST)/ccache
+  $(if $(CONFIG_CCACHE),$(1) : export CCACHE_DIR:=$(STAGING_DIR_HOST)/ccache)
   $(if $(HOST_CONFIG_SITE),$(1) : export CONFIG_SITE:=$(HOST_CONFIG_SITE))
   $(if $(IS_PACKAGE_BUILD),$(1) : export PATH=$$(TARGET_PATH_PKG))
 endef

include/package.mk

@@ -174,9 +174,7 @@ define Build/Exports/Default
   $(1) : export CONFIG_SITE:=$$(CONFIG_SITE)
   $(1) : export PKG_CONFIG_PATH:=$$(PKG_CONFIG_PATH)
   $(1) : export PKG_CONFIG_LIBDIR:=$$(PKG_CONFIG_PATH)
-# do not set CCACHE_dir. We want to use the homedirecoty of the user.
-# otherwise nigtly and new builds will not see a speedup.
-#  $(1) : export CCACHE_DIR:=$(STAGING_DIR)/ccache
+  $(if $(CONFIG_CCACHE),$(1) : export CCACHE_DIR:=$(STAGING_DIR)/ccache)
 endef
 Build/Exports=$(Build/Exports/Default)