scripts/config: add option to undef a symbol

It is currently possible to enable, disable or modularise
a symbol. Also, an undefined symbol is reported as such.

Add a new command to undefine a symbol, by removing the
corresponding line from the .config file.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>