Use bash for the shell

- soc.mak uses some bashisms, such as source. Set the SHELL variable to
/bin/bash to force using bash.

Change-Id: If79a04fd5d3080003505805dbcffef584406f88b
diff --git a/iMX8M/soc.mak b/iMX8M/soc.mak
index ef545c7..21c4ba8 100644
--- a/iMX8M/soc.mak
+++ b/iMX8M/soc.mak
@@ -1,4 +1,5 @@
 #Copyright 2018 NXP
+SHELL := /bin/bash
 
 OUTIMG = flash.bin
 DCD_CFG_SRC = imx8mq_dcd.cfg