revert the change in gbuild.mk : use /bin/sh as SHELL

This is for performance reasons.
there is a measurable benefit of being able to use /bin/dash
vs /bin/bash
and since make is a big consumer of shells, it is worth the pain.

Change-Id: I0654bee1e7bab1041cd00b5e71498bd593aeeecb

solenv/gbuild/gbuild.mk

@@ -48,7 +48,7 @@ GBUILDDIR:=$(SRCDIR)/solenv/gbuild
 
 .DELETE_ON_ERROR:
 
-SHELL := /usr/bin/env bash
+SHELL := /bin/sh
 true := T
 false :=
 define NEWLINE