fix bugs in mkimg script and make the default image /n/atom/ftp/+usbinstamd64.bz2
this should be changed to /n/atom/ftp/usbinstamd64.bz2 when the script is trusted.

Reference: /n/atom/patch/applied2013/mkimgplus
Date: Tue Dec 24 00:26:14 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/lib/dist/amd64/mkimg	Tue Dec 24 00:25:36 2013
+++ /sys/lib/dist/amd64/mkimg	Tue Dec 24 00:25:36 2013
@@ -15,9 +15,9 @@
 }
 
 imgroot=/n/mkimg
-~ $# flagm 0 && flagm = mkboot
-~ $# flago 0 && {~ $flagm mkboot && flago = usbtest.bz2 || flago = usbinstamd64.bz2.bz2}
-~ $# flags 0 && flags = 500
+~ $#flagm 0 && flagm = mkboot-install
+~ $#flago 0 && {~ $flagm mkboot && flago = usbtest.bz2 || flago = /n/atom/ftp/+usbinstamd64.bz2}
+~ $#flags 0 && flags = 500	#default 500mb
 
 ramfs -m $imgroot			|| fail
 dd -if /dev/zero -of $imgroot/data -bs 1m -oseek `{echo $flags - 1 | hoc} -count 1 || fail