buildroot

infra: introduce suitable-extractor helper function

In order to simplify determining the right extractor tool for a given
file type, this patch introduces a make function 'suitable-extractor'.
Its usage is $(call suitable-extractor,filename), and it returns the
path to the suitable extractor.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>