Clear the DDR memory. This should be done by bootloaders,

but they have no such feature yet.

This fixes operation on Rocket Core and lowRISC.

sys/conf/options.riscv

@@ -2,3 +2,4 @@
 
 RISCV				opt_global.h
 VFP				opt_global.h
+DDR_CLEAR_SIZE			opt_global.h

sys/riscv/riscv/locore.S

@@ -126,6 +126,17 @@ _start:
 	csrr	a0, mhartid
 	bnez	a0, mpentry
 
+#if defined(DDR_CLEAR_SIZE)
+	/* Clear DDR memory */
+	la	t0, _end
+	li	t1, DDR_CLEAR_SIZE
+1:
+	sd	zero, 0(t0)
+	addi	t0, t0, 8
+	bltu	t0, t1, 1b
+	/* End */
+#endif
+
 	/* Build event queue for current core */
 	build_ring