tools build: Add detected config support

Adding support to include detected configuration makefile into the build
process. This will allow the Build objects to be configurable based on
the config data, like:

  perf-$(CONFIG_KRAVA) += krava.o

The configuration is stored in '.config-detected' file, which is
generated for each compilation.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Cc: Alexis Berlemont <alexis.berlemont@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-bl8qho0ubck7aqrbbfu9inlm@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/build/Makefile.build

@@ -22,6 +22,9 @@ build-dir := $(srctree)/tools/build
 # Generic definitions
 include $(build-dir)/Build.include
 
+# do not force detected configuration
+-include .config-detected
+
 # Init all relevant variables used in build files so
 # 1) they have correct type
 # 2) they do not inherit any value from the environment

tools/perf/config/Makefile

@@ -11,6 +11,10 @@ ifneq ($(obj-perf),)
 obj-perf := $(abspath $(obj-perf))/
 endif
 
+$(shell echo -n > .config-detected)
+detected     = $(shell echo "$(1)=y"       >> .config-detected)
+detected_var = $(shell echo "$(1)=$($(1))" >> .config-detected)
+
 LIB_INCLUDE := $(srctree)/tools/lib/
 CFLAGS := $(EXTRA_CFLAGS) $(EXTRA_WARNINGS)