doc: develop: Describe system configuration

Start by describing in general the best practices for how to implement
configuration of some aspect of U-Boot.  This generally means finding
the right choices for when something should be static or dynamically
configured and enabled.  Then further document when to use CONFIG or CFG
namespaces for static configuration.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>