Replace MBEDTLS_EINTR by IS_EINTR
Replace MBEDTLS_EINTR by IS_EINTR

check-names.sh reserves the prefix MBEDTLS_ for macros defined in
config.h so this name (or check-names.sh) had to change.

This is also more flexible because it allows for platforms that don't have
an EINTR equivalent or have multiple such values.

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