abuild: Use 12 lines of context for errors

The current default of 6 lines leaves us with no context
about the actual error:

*** ERROR: 3 warnings encountered, and warnings are errors.

coreboot-gerrit/util/kconfig/Makefile:38: recipe for target 'oldconfig' failed
make[1]: *** [oldconfig] Error 1
make[1]: Leaving directory 'coreboot-gerrit'

Change-Id: I67e7d740e7b3b1c66005dc1bf50557a20bc15428
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: https://review.coreboot.org/12720
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

util/abuild/abuild

@@ -40,7 +40,7 @@ if [ -d "$XGCCPATH" ] && [[ ":$PATH:" != *":$XGCCPATH:"* ]]; then
 fi
 
 # Lines of error context to be printed in FAILURE case
-CONTEXT=6
+CONTEXT=12
 
 # Configure-only mode
 configureonly=0