tools: Fix freeing of image memory after cell creation

Regression of 95666fd1: We are not allocating multiple images, and the
image variable does not point to a valid address when we try to free the
former only image at the end of cell_create. Properly loop over all the
images for freeing.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
gitignore: Exclude vi temp files

Do not track temporary files created by vi.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Documentation: a few cosmetic changes

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>