cmd: add rng command

For the RNG uclass we currently only have a test working on the sandbox.

Provide a command to test the hardware random number generator on
non-sandbox systems.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/cmd/Makefile b/cmd/Makefile
index 12e898d..8df39f3 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -117,6 +117,7 @@
 obj-$(CONFIG_CMD_REGINFO) += reginfo.o
 obj-$(CONFIG_CMD_REISER) += reiser.o
 obj-$(CONFIG_CMD_REMOTEPROC) += remoteproc.o
+obj-$(CONFIG_CMD_RNG) += rng.o
 obj-$(CONFIG_CMD_ROCKUSB) += rockusb.o
 obj-$(CONFIG_SANDBOX) += host.o
 obj-$(CONFIG_CMD_SATA) += sata.o