Create a separate header file (mbedtls/build_info.h) to use when
depending on the config options defined in config.h.
Also copy the handling of the MBEDTLS_CONFIG_FILE macro into the new
header, so that the next commit can remove this code from every other
place where config.h used to be included.
Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
