sparc: documentation about the boot process and useful commands

I didn't do anything with sparc for a few weeks (you don't want this
machine running when temperatures already are over 30°...), and I wastd
some time finding back some of the useful information, such as commands
to boot and debug, load and execution address of the bootloader program,
etc. So let's keep these in the documentation directory.

Change-Id: I293e0eea3063d410d66f9b2397c2cf0bdbfc6753
Reviewed-on: https://review.haiku-os.org/c/1581
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

docs/develop/kernel/arch/sparc/openboot.txt

@@ -6,8 +6,13 @@ Executable format
 
 PowerPC uses COFF. Sparc uses a.out, which is a lot simpler. According to the
 spec, some fields should be zeroed out, but they say implementation may chose
-to allow other values, so it's possible a standard a.out file would work as
-well.
+to allow other values, so a standard a.out file works as well.
+
+It used to be possible to generate one with objcopy, but support was removed,
+so we now use elf2aout (imported from FreeBSD).
+
+The file is first loaded at 4000, then relocated to its load address (we use
+202000 and executed there)
 
 Openfirmware prompt
 ===================
@@ -47,7 +52,17 @@ Boot from network
 -----------------
 
 rarp:
-boot net
+boot net:,somefile
+(net is an alias to the network card and also sets the load address: /pci@1f,4000/network@1,1)
 
 dhcp:
 boot net:dhcp
+
+Debugging
+---------
+
+202000 dis (disassemble starting at 202000 until next return instruction)
+
+4000 1000 dump (dump 1000 bytes from address 4000)
+
+ctrace (backtrace)