Don't use ninja to build targets.

Ninja does not preserve the -e option given to make.

Change-Id: I3b540aa420a57369b67565477a31c6ff2c17afaf
diff --git a/tools/buildbot-build.sh b/tools/buildbot-build.sh
index eb64994..a670fc7 100755
--- a/tools/buildbot-build.sh
+++ b/tools/buildbot-build.sh
@@ -76,6 +76,8 @@
   if [[ $TARGET_PRODUCT == "mips32r2_fp" ]]; then
     env="$env USE_CLANG_PLATFORM_BUILD=true"
   fi
+  # Disable NINJA for building on target, it does not support the -e option to Makefile.
+  env="$env USE_NINJA=false"
   # Use '-e' to force the override of TARGET_GLOBAL_LDFLAGS.
   # Also, we build extra tools that will be used by tests, so that
   # they are compiled with our own linker.