memtester: convert to gentargets and bump to 4.2.1

Also, memtester requires largefile support

[ Thomas: add patch to fix memtester's Makefile at installation
  stage. Bump to 4.2.1. ]

Signed-off-by: Martin Banky <Martin.Banky@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>