config: improve help text and prompt for debugging related options

The help text for the choice of different stripping levels is removed,
since it is not displayed by menuconfig. Instead, only the per-option
help text is visible, so this text is improved.

[Peter: slightly reworked text]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>