external/mambo: Add an option to exit Mambo when the system is shutdown

Automatically exiting can be convenient for scripting. Will also exit
due to a HW crash (eg. unhandled exception).

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[stewart: handle case where SKIBOOT_AUTORUN is not set]
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

external/mambo/README.md

@@ -40,6 +40,11 @@ export SKIBOOT_ZIMAGE=$HOME/src/op-build/output/images/zImage.epapr
 export SKIBOOT=$HOME/src/op-build/output/images/skiboot.lid
 export SKIBOOT_AUTORUN=1
 ```
+If you want Mambo to autmatically run *AND* exit when the system is
+shutdown (or has a HW crash) use this:
+```
+export SKIBOOT_AUTORUN=2
+```
 If you want a vmlinux and separate initramfs you can also do this:
 ```
 export SKIBOOT_ZIMAGE=$HOME/src/op-build/output/images/vmlinux

external/mambo/skiboot.tcl

@@ -590,3 +590,7 @@ if { [info exists env(SKIBOOT_AUTORUN)] } {
 } else {
 	readline
 }
+
+if { [info exists env(SKIBOOT_AUTORUN)] && $env(SKIBOOT_AUTORUN) == 2 } {
+    quit
+}