releasetools: fix avbtool arguments.

The following error occurred when building dist.
Separate these arguments

error log:
avbtool: error: unrecognized arguments: --partition_name vendor_boot

Bug: 144820089

Change-Id: I3891ef3bd9a2f7d2cd1377228b9894b599dc1cc3
Signed-off-by: Donghoon Yu <hoony.yu@samsung.com>