image.bbclass: Fix 'vardepsexclude' mechanism for image_cmd_${FSTYPE}

image.bbclass: Fix 'vardepsexclude' mechanism for image_cmd_${FSTYPE}

Current mechanism doesn't allow to use any non-determenistic variable, except 'DATE' and 'DATETIME', inside IMAGE_CMD_${FSTYPE} prototype.

Passing 'vardepsexclude' values from IMAGE_CMD_${FSTYPE}, so users will be able to avoid taskhash mismatch problems.

Signed-off-by: Igor Romanov <i.romanov@inango-systems.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Armin Kuster <akuster@mvista.com>