Add command to determine EXAMPLE_LIST

Signed-off-by: Firas Zaidan <firas@zaidan.de>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/Makefile b/Makefile
index a00dc38..f870a61 100644
--- a/Makefile
+++ b/Makefile
@@ -2,10 +2,7 @@
 
 OUTPUT_DIR := $(CURDIR)/out
 
-EXAMPLE_LIST := hello_world
-EXAMPLE_LIST += random
-EXAMPLE_LIST += aes
-EXAMPLE_LIST += hotp
+EXAMPLE_LIST := $(subst /,,$(dir $(wildcard */Makefile)))
 
 .PHONY: all
 all: examples prepare-for-rootfs