base: Make SHELL point to bash

On some weirdly strict environments, SHELL in make is set to /bin/sh
instead of bash. This breaks soe of the clean conventions we use.

Change-Id: I0bc4264397060789c60eab47f30aa91e9c8b6dbd
diff --git a/Makefile b/Makefile
index e361afe..f51a692 100644
--- a/Makefile
+++ b/Makefile
@@ -1,3 +1,5 @@
+SHELL := $(shell which /bin/bash)
+
 ifeq ($(ROOTDIR),)
 $(error $$ROOTDIR IS NOT DEFINED -- don\'t forget to source setup.sh)
 endif