Improve the description of EXCLUDED_FROM_FULL
Improve the description of EXCLUDED_FROM_FULL

Every boolean (defined/undefined) symbol is a "variant toggle" in some
sense, even enabling a module with MBEDTLS_xxx_C. What matters is
whether the symbol influences some other part of the system in such a
way that we need to run tests separately with and without it being
defined.

Signed-off-by: Gilles Peskine <f805f64266d288fc5467baa7be6cd0ff366f477b@arm.com>

Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls