build: add PLATFORM_FLAVOR for BUILD_OPTEE_OS
build: add PLATFORM_FLAVOR for BUILD_OPTEE_OS

fix build error:
In file included from core/arch/arm/kernel/thread.c:29:0:
core/arch/arm/plat-aquila/./platform_config.h:51:2: error: #error "Unknown platform flavor"
 #error "Unknown platform flavor"

Signed-off-by: Zhizhou Zhang <ca4dd25998089748c1b2c88027b9c671f48f7fc5@gmail.com>
Reviewed-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>

matt2048/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,matt2048/optee_os,matt2048/optee_os,matt2048/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os