cpu/riscv_common: Jump to rom start on boot

With this the riscv start code jumps to the ROM start on boot when the
ROM area doesn't start at address 0x0.

cpu/riscv_common/Makefile.include

@@ -21,6 +21,8 @@ ifneq (,$(ITIM_START_ADDR))
   LINKFLAGS += $(LINKFLAGPREFIX)--defsym=_itim_length=$(ITIM_LEN)
 endif
 
+CFLAGS += -DROM_START_ADDR=$(ROM_START_ADDR)
+
 LINKER_SCRIPT ?= riscv.ld
 
 include $(RIOTMAKE)/arch/riscv.inc.mk

cpu/riscv_common/start.S

@@ -5,7 +5,10 @@
  * General Public License v2.1. See the file LICENSE in the top level
  * directory for more details.
  */
-    .section .init
+#include "vendor/riscv_csr.h"
+
+.section .init
+
     .globl _start
     .type _start,@function
 
@@ -14,6 +17,15 @@ _start:
     .cfi_undefined ra
 .option push
 .option norelax
+    csrc CSR_MSTATUS, MSTATUS_MIE
+    la a0, _start
+    li a1, ROM_START_ADDR
+    bleu a1, a0, _start_real
+    la a0, _start_real
+    add a0, a0, a1
+    jr a0
+
+_start_real:
     la gp, __global_pointer$
 .option pop
     la sp, _sp