Makefile: use plain bash for SHELL

Some linux distributions do not provide bash at the /bin/bash location,
use bash directly and expect the user to setup a proper $PATH.

Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/Makefile b/Makefile
index 474407e..571fba7 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-SHELL = /bin/bash
+SHELL = bash
 
 # It can happen that a makefile calls us, which contains an 'export' directive
 # or the '.EXPORT_ALL_VARIABLES:' special target. In this case, all the make