change aarch64 TFTP locations from pxelinux/ to aarch64/

This commit also removes the symlink creation/removal business, which is
not needed. Instead we can just write a default config to grub.cfg
which immediately exits. This achieves the same effect and more closely
corresponds to how we handle PXELINUX.

Bug: 1078965
Change-Id: Id7bd78c0a59f3ce8d60965f682bee3f502e4883f
tests for beaker-pxemenu

Change-Id: Id01dfb6c85dbb1d62e3c4dad1fafc7fba6310d96
generate menu for aarch64 in beaker-pxemenu

Bug: 1084188
Change-Id: I4787af7e8fec625f831fe32da96708dfff6779fa