Allow use of alternative to bash as the build shell.

Change-Id: Ib92217f90c98e04a1d16efd4904d9d7ba9dda3cf
http://b/issue?id=2557214
diff --git a/core/main.mk b/core/main.mk
index b1a5bff..3da799b 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1,8 +1,11 @@
-
+ifdef ANDROID_BUILD_SHELL
+SHELL := $(ANDROID_BUILD_SHELL)
+else
 # Use bash, not whatever shell somebody has installed as /bin/sh
 # This is repeated in config.mk, since envsetup.sh runs that file
 # directly.
 SHELL := /bin/bash
+endif
 
 # this turns off the suffix rules built into make
 .SUFFIXES: