tools: imx8m_image: fix warning message

When a firmware file is missing the warning message doesn't indicate the
firmware file name because '$tmp' var doesn't exist.
Fix the warning message and while at it reduce the if/else statement.

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>