ccache: allow for BR2_CCACHE_DIR environment override

Allow the BR2_CCACHE_DIR .config option to be overriden by the
BR2_CCACHE_DIR env variable.
This is useful for big projects where in some cases the developers home
directory might be a NFS mount (slow) and real production builds aren't.

Update documentation accordingly as well.

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -183,7 +183,9 @@ endif
 ifneq ($(BR2_DL_DIR),)
 DL_DIR := $(BR2_DL_DIR)
 endif
-
+ifneq ($(BR2_CCACHE_DIR),)
+BR_CACHE_DIR := $(BR2_CCACHE_DIR)
+endif
 
 # Need that early, before we scan packages
 # Avoids doing the $(or...) everytime
@@ -371,7 +373,7 @@ TARGET_DIR_WARNING_FILE = $(TARGET_DIR)/THIS_IS_NOT_YOUR_ROOT_FILESYSTEM
 
 ifeq ($(BR2_CCACHE),y)
 CCACHE := $(HOST_DIR)/usr/bin/ccache
-BR_CACHE_DIR = $(call qstrip,$(BR2_CCACHE_DIR))
+BR_CACHE_DIR ?= $(call qstrip,$(BR2_CCACHE_DIR))
 export BR_CACHE_DIR
 HOSTCC := $(CCACHE) $(HOSTCC)
 HOSTCXX := $(CCACHE) $(HOSTCXX)

docs/manual/common-usage.txt

@@ -84,6 +84,9 @@ to +make+ or set in the environment:
   configuration interface, so through the Buildroot +.config+ file; this
   is the recommended way of setting it.
   +
+* +BR2_CCACHE_DIR+ to override the directory where
+  Buildroot stores the cached files when using ccache.
+  +
 * +BR2_DL_DIR+ to override the directory in which
   Buildroot stores/retrieves downloaded files
   +