tests: framework add tst_strerr()

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
tests: filters: Fix Makefile.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: Move GP_Loader structures to loaders

This commit moves the GP_Loader structures from GP_Loader.c to
respecitive loaders which is preparation for additional members
that will be added to GP_Loader structure.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: GP_Loader: Get rid of unneeded members

This commit cleanups a few aspects of loaders implementation.

It creates GP_LoaderLoadImage() function that can be used to load
image from path for a given loader pointer which allows us to.

* Get rid of the Load() callback from GP_Loader

* Unify GP_LoadFOO implementation

Signed-off-by: Cyril Hrubis <metan@ucw.cz>