serial_putchar() always hung if it was called and the serial port existed,
so booting with -h hung when the above bug was fixed. Previously, setting
-h did nothing but -h was sometimes the default due to the stack garbage
bug.
Submitted by: DI. Christian Gusenbauer <cg@scotty.edvz.uni-linz.ac.at>
The `howto' arg to boot() was not supplied, so it was stack garbage (actually
the return address in the boot program). I didn't use the submitted fix.
