Simplify platform testing macros

Update the main Makefile so that PLATFORM_$(PLATFORM) and
PLATFORM_FLAVOR_$(PLATFORM_FLAVOR) are set to 'y', and add these
variables to the export list for the generation of conf.h.

As a result, the definition of numerical flavor identifiers in the
multiple platform_config.h files is not needed anymore, and we can also
get rid of the PLATFORM_FLAVOR_IS() test macro. Instead, replace all
occurrences of '#if PLATFORM_FLAVOR_IS(foo)' with
'#if defined(PLATFORM_FLAVOR_foo)'.

This makes it possible to test the platform and not only the flavor in
any source file, so drop the manual definition of PLATFORM_hikey.

Finally, remove the definitions of platform_$(PLATFORM) and
platform_flavor_$(PLATFORM_FLAVOR) from core/core.mk since they are not
used.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: David Brown <david.brown@linaro.org>
diff --git a/Makefile b/Makefile
index 7911989..fb96c68 100644
--- a/Makefile
+++ b/Makefile
@@ -26,6 +26,8 @@
 PLATFORM_FLAVOR ?= qemu_virt
 O		?= out/$(ARCH)-plat-$(PLATFORM)
 
+PLATFORM_$(PLATFORM) := y
+PLATFORM_FLAVOR_$(PLATFORM_FLAVOR) := y
 arch_$(ARCH)	:= y
 
 ifneq ($O,)