This addes the option to treat recursive dependencies as warnings
instead of errors, by running make with WARN_RECURSIVE_DEP=1.

Note that the script/config targets will not get rebuilt when you add or
remove WARN_RECURSIVE_DEP while running make.  One must run
'make config-clean' before building config with a different setting.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>