Allow running /somewhere/else/path/to/abi_check.py
Allow running /somewhere/else/path/to/abi_check.py

Don't require abi_check.py to be the one in scripts/ under the current
directory.

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