ignore optee_os make clean error codes

optee_os version 2.3.0 changes the way "make clean" works.

This commit updates the scripts to cope with the change by ignoring the
return codes from the build.

Change-Id: Iaeadfe5765669df7cfb8266fb728e7ab08000162
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>