build: Allow overriding BUILDDIR from command line

You can now specify BUILDDIR= on the make command line,
e.g., in order to put that into a tmpfs or similar.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

src/build.rules

@@ -7,7 +7,8 @@ all: _all
 # setup some variables
 ROOTDIR := $(dir $(lastword $(MAKEFILE_LIST)))
 ROOTDIR := $(dir $(ROOTDIR:%../src/=%))../
-BUILDDIR := $(ROOTDIR)build
+BUILDDIR ?= $(abspath $(ROOTDIR)build)
+BUILDDIR := $(BUILDDIR:%/=%)
 _PROJ := $(abspath $(dir $(firstword $(MAKEFILE_LIST))))
 ABSROOT := $(abspath $(ROOTDIR))
 _PROJ := $(_PROJ:$(ABSROOT)/%=%)