usage: fastboot [ <option> ] <command>
update <filename> reflash device from update.zip
flashall 'flash boot' + 'flash system'
flash <partition> [ <filename> ] write a file to a flash partition
erase <partition> erase a flash partition
getvar <variable> display a bootloader variable
boot <kernel> [ <ramdisk> ] download and boot kernel
flash:raw boot <kernel> [ <ramdisk> ] create bootimage and flash it
devices list all connected devices
reboot reboot device normally
reboot-bootloader reboot device into bootloader
-w erase userdata and cache
-s <serial number> specify device serial number
-p <product> specify product name
-c <cmdline> override kernel commandline
I think it's safe to assume that the options listed here are the options you have available to you when running the
fastboot binary. Since there doesn't seem to be an override as a native function of
fastboot, your only solution would be to access the source code of
fastboot (if available) and remove or alter the part of the code where the device reboots.
Note that there might be very good reasons as to why the device reboots at that stage, so I'd be careful with messing around with it.