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.
This commit fixes the case when new Kconfig configuration options were added and you run `make menuconfig`.
Previously this would first show console prompt to set values for new options, and then launch menuconfig.
Now this will just launch menuconfig.
New target is added, defconfig, which may be useful for non-interactive environments to set default values for
all new configuration options.
Angus Gratton