mkbootimg: also deal with signedness on dt_size
libmincrypt: suppress warning messages

- deal with implict declaration issue

Change-Id: I253f1412a696c4811ade70a931fb1c01b31c62a5
Remove declaration of unused static variable

The declaration is useless and triggers a warning with clang 3.5 (in
code that's built with -Werror)

Change-Id: I8fc42178df264509dcce6e1f1ba318bf849307e3
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
Makefile: update clean; simplify .gitignore