Fix invalid default value of $(PLATFORM_FLAVOR)

Fixes the following error:

  $ make -s PLATFORM=stm
  core/arch/arm/plat-stm/system_config.mk:20:
      *** PLATFORM_FLAVOR=qemu_virt is not supported.  Stop.

(one would expect PLATFORM_FLAVOR to default to 'orly2').
This patch does the following:

- Do not set "PLATFORM_FLAVOR ?= qemu_virt" in the main Makefile. The
default value for PLATFORM_FLAVOR has to be defined by each platform in
plat-*/conf.mk.
- Consistent with the above, "PLATFORM_FLAVOR_$(PLATFORM_FLAVOR) := y"
is moved to core/core.mk, after $(platform-dir)/conf.mk has been
included, i.e., where $(PLATFORM_FLAVOR) can be relied upon.
- All definitions of "PLATFORM_FLAVOR_$(PLATFORM_FLAVOR) := y"
are removed from platform files, since it is already taken care of in
core/core.mk.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/Makefile b/Makefile
index fb96c68..f74b93f 100644
--- a/Makefile
+++ b/Makefile
@@ -23,11 +23,9 @@
 # Make these default for now
 ARCH            ?= arm
 PLATFORM        ?= vexpress
-PLATFORM_FLAVOR ?= qemu_virt
+# Default value for PLATFORM_FLAVOR is set in plat-$(PLATFORM)/conf.mk
 O		?= out/$(ARCH)-plat-$(PLATFORM)
 
-PLATFORM_$(PLATFORM) := y
-PLATFORM_FLAVOR_$(PLATFORM_FLAVOR) := y
 arch_$(ARCH)	:= y
 
 ifneq ($O,)