This is related to https://github.com/ARMmbed/mbed-os/issues/11256
When TARGET_PSA is enabled on Cortex-A boards, the SECURE
is redeclared. So to minimize the impact we can redeclare this in
val.h which is used only by Mbed. And `security_t` itself is used in
mbed-os\components\TARGET_PSA\TESTS, but the SECURE string is not used.
Renamed to `caller_security_t` which aligns with
https://github.com/ARM-software/psa-arch-tests