makefile: new qstrip make function

Define a qstrip function that strips double quotes and then
whitespaces around a given string. This is a very commonly needed
thing, and having a function allows to remove a large quantity of
hand-written definitions, followed by a dummy comment to make text
editors not crazy because of the unmatching double quote.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>