core: fix gen_tee_bin.py to handle STB_LOCAL symbols

Prior to this patch scripts/gen_tee_bin.py only looked for global
symbols (STB_GLOBAL). The linker in some older versions of the gcc
toolchain makes some of the symbols local (STB_LOCAL) instead. This
patch fixes that by falling back to a local symbol in case a global
cannot be found.

Reviewed-by: Jerome Forissier <jerome@forissier.org>
Reported-by: Victor Chong <victor.chong@linaro.org>
Fixes: 3c51966baa03 ("core: add scripts/gen_tee_bin.py for boot binaries")
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>