tee-supplicant: make RPMB_EMU a conditional assignment

At the moment the RPMB_EMU variable in the Makefile uses a simple
assignment and unconditionally sets the variable.
Move it to a conditional assignment and allow users to override it
from the command line with:
CROSS_COMPILE=aarch64-linux-gnu- RPMB_EMU=0 make

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>