Dotconfig: Single entry point for all options

[danielinux]

The goal of this feature is to introduce a new configuration layer, on top of the existing Makefile environment.

• include/target.h and the linker scripts for wolfboot and the test application are now automatically generated at compile time according to the global configuration options for the flash geometry.

• An optional .config file can now be provided to alter the default options. Custom make invocation that alter the environment still have priority on the values stored in .config.

• A new target make config shows an interactive prompt to create a (complete) custom .config file.

• A few .config file usage examples are provided under config/examples

test.mk has been updated accordingly, all test passing on the CI testbed.

Ready for review.

[dgarske]

Thanks Daniele! Excellent feature addition... make config.