boot-qemu.py: Set default m68k memory to 512m

The default switch to 1G ends up causing panics on boot from memory
allocation issues, at least with the default mac_defconfig. Given the
real Mac machines this is modeled after had less memory than this and
QEMU only allows up to 1G, it is likely some code path is not well
tested.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: nathanchance

boot-qemu.py

@@ -528,6 +528,7 @@ def __init__(self):
             '-M', 'q800',
             '-no-reboot',
         ]  # fmt: off
+        self.memory = '512m'
 
 
 class MIPSQEMURunner(QEMURunner):