[cmake]: Change to shorter RKPLATFORM macro

Change Android and Linux branches to use RKPLATFORM macro.
RKPLATFORM is shorter and it is not a cmake variable.

Change-Id: I09a6fe72e7dacedbb4b2b8e1a66b6f7ca625f019
Signed-off-by: Herman Chen <herman.chen@rock-chips.com>

build/linux/arm/make-Makefiles.bash

@@ -23,6 +23,6 @@ fi
 # generate Makefile
 cmake -DCMAKE_BUILD_TYPE=Release \
       -DCMAKE_TOOLCHAIN_FILE=./arm.linux.cross.cmake \
-      -DCMAKE_RKPLATFORM_ENABLE=ON \
+      -DRKPLATFORM=ON \
       -G "Unix Makefiles" \
       ${MPP_TOP}