Contribution description

Now that we have the libsdl2-dev:amd64 package in the CI image (RIOT-OS/riotdocker#241), we can remove native64 from the blacklist and build the lvgl tests in the CI.

The CI doesn't run the GUI program, but if we run the executable directly, we can see that the lvgl heap size chosen for RIOT is too small for a 64-bit system, since lvgl allocates a lot of space for pointers and the current limits seem to be as small as possible to make the examples run on 32-bit platforms.
I changed the default lvgl heap sizes in pkg/lvgl/include/lv_conf.h for 64-bit systems so that the examples can run.

The correct heap size is very application and platform specific. Simply extending the header to include a variety of different default heap sizes to run the examples may not be the most elegant way to go about it.
Another option include:

• set different heap sizes in the Makefile.
• set a larger default in board.h for some boards
• use LV_MEM_CUSTOM
• choose a larger default heap size (such as the recommended >48 kB), which would fix most run-time crashes, but would probably result in quite a few build failures due to insufficient memory.

If you have an opinion on how to handle the default heap size for lvgl, let me know.

Testing procedure

Check that you have the libsdl2-dev:amd64 package installed and run the tests, e.g.:

• BOARD=native64 make all term -C tests/pkg/lvgl
• BOARD=native64 make all term -C tests/pkg/lvgl_touch