cmd: pxe: Increase maximum path length

On NixOS, cross compiled kernels have long suffixes that cause them to
exceed the current maximum path length. The PXE/TFTP max path length is
used for extlinux.conf support as well, which is where this problem
usually manifest's itself.

Signed-off-by: Ben Wolsieffer <benwolsieffer@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>