distro_boot: Fix block device after DHCP boot sequence

The generic distro boot sequence iterates over several types of devices,
trying one after another. In doing do, it starts with setting the
"devtype" variable, then uses this later in more generic commands.

Now most (block) device types use a local variable for that, but DHCP
puts the type into the environment, where it shadows any local variables.
As a consequence any boot attempt after bootcmd_dhcp has been run fails:
===========================
VExpress64# run bootcmd_dhcp
...
VExpress64# run bootcmd_sata0
SATA#0:
        (3.0 Gbps)
SATA#1:
        (No RDY)
Device 0: Model: 16GB SATA Flash Drive  ....
... is now current device
Couldn't find partition dhcp 0:1
===========================
This problem typically doesn't show, because DHCP is mostly the last
command to try, but is a problem when this order is different, or when
distro_bootcmd or bootcmd_xxx are run separately or multiple times.

Let bootcmd_dhcp use a local variable, as the other kids do, to make the
order of boot commands irrelevant, and allow repeated calls.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>