Change variable name to lowercase
Change variable name to lowercase

This seems more correct for variables that are only used in
a small area of the cmake file.

Signed-off-by: David Horstmann <0386b5e8f8e5ea8a58d40026a5369df20e1c750d@arm.com>

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