uboot-imx

Makefile: drop mention of *.cfgtmp

Since commit f916757300 (imx: Create distinct pre-processed mkimage
config files), *.cfgtmp files are no longer generated. There is no need
to remove them on the 'clean' target anymore.

Rename the .gitignore glob to *.cfgout.

Cc: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>