hotp: Initial commit for HMAC-OTP (RFC4226)

Add host application and a new TA that are capable of generating HMAC
based One Time Passwords according to the RFC4226 specification [1].

[1] https://www.ietf.org/rfc/rfc4226.txt

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (QEMU v7)
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/Makefile b/Makefile
index bc92298..a00dc38 100644
--- a/Makefile
+++ b/Makefile
@@ -5,6 +5,7 @@
 EXAMPLE_LIST := hello_world
 EXAMPLE_LIST += random
 EXAMPLE_LIST += aes
+EXAMPLE_LIST += hotp
 
 .PHONY: all
 all: examples prepare-for-rootfs