distro_bootcmd: read DHCP boot script name from a variable

Modify $bootcmd_dhcp to read the downloaded script filename from an
environment variable rather than hard-coding it. This allows the user
(or another script) to select a different script name if they want,
without editing the whole value of $bootcmd_dhcp.

Signed-off-by: Stephen Warren <swarren@nvidia.com>