[form] Add support for dynamically created interactive forms

[form] Add support for dynamically created interactive forms

Add support for presenting a dynamic user interface as an interactive
form, alongside the existing support for presenting a dynamic user
interface as a menu.

An interactive form may be used to allow a user to input (or edit)
values for multiple settings on a single screen, as a user-friendly
alternative to prompting for setting values via the "read" command.

In the present implementation, all input fields must fit on a single
screen (with no scrolling), and the only supported widget type is an
editable text box.

Signed-off-by: Michael Brown <mcb30@ipxe.org>