core: create conf.h from CFG_* Makefile variables

Simplify the use of makefile configuration variables from C code.
With this patch, one can #include <generated/conf.h> instead of adding
CPP flags definitions to the .mk files.

- CFG_* variables that are set to 'y' are converted to: #define CFG_FOO 1
- Undefined variables, or variables set to 'n' remain undefined in conf.h
- CFG_* variables with any other value are output unchanged

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>