Simplifies examples of embedding a certificate file or a root cert.
This is a much cruder mechanism than the full flash filesystem we want
eventually, but still sometimes useful.
Used by mbedTLS to set MBEDTLS_CONFIG_FILE in all components.
This change sets CFLAGS/etc at the project level and then exports those
variables for components, rather than setting them independently each time
a component Makefile is invoked.
