yoda: Remove 'depends' line from code

Not yet clear how to handle it.

Signed-off-by: Pavel Emelyanov <xemul@sacred.ru>
yoda: Add req_for keyword

This one denotes, that the option in question is required for
anything. After the kwd there should go a string describing what
the option is required for.

The rt requirement check is -- if the expression is true and the
option is not provided, an error is reported.

Signed-off-by: Pavel Emelyanov <xemul@sacred.ru>
yoda: Simple expressions analyzer

Understands expressions of form

  <exp>(|<exp>)*

where exp is

  <opt_name>[=<opt_val>]

Enough for the proof-of-concept.

Signed-off-by: Pavel Emelyanov <xemul@sacred.ru>