bootrom: also setup SBI a0+a1 for when we hang (#617)

This way a program started by the debugger still has the DTB handy.

bootrom/bootrom.S

@@ -1,7 +1,9 @@
+#define DRAM_BASE 0x80000000
+
 .section .text.start, "ax", @progbits
 .globl _start
 _start:
-  la s0, DRAM_BASE
+  li s0, DRAM_BASE
   csrr a0, mhartid
   la a1, _dtb
   jr s0
@@ -9,8 +11,11 @@ _start:
 .section .text.hang, "ax", @progbits
 .globl _hang
 _hang:
+  csrr a0, mhartid
+  la a1, _dtb
+1:
   wfi
-  j _hang
+  j 1b
 
 .section .rodata.dtb, "a", @progbits
 .globl _dtb

bootrom/linker.ld

@@ -1,7 +1,6 @@
 SECTIONS
 {
-    DRAM_BASE = 0x80000000;
-    ROM_BASE = 0x10000;
+    ROM_BASE = 0x10000; /* ... but actually position independent */
 
     . = ROM_BASE;
     .text.start : { *(.text.start) }