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>
diff --git a/.gitignore b/.gitignore
index 6bb0ade..8d18d6f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,7 +8,7 @@
 .*
 *.a
 *.bin
-*.cfgtmp
+*.cfgout
 *.dtb
 *.dtb.S
 *.elf