treewide: provide consistent basic DEVICE_TYPE

treewide: provide consistent basic DEVICE_TYPE

While the effective "default" based on frequent use is "router", the
DEVICE_TYPE variable actually provides a "basic" configuration without
selecting any additional packages.

This is currently set up with the identifier "bootloader", which seems
to be not used at all. However, the only targets not using "router" or
"nas" are actually archs38 and arc770, which use their own value
"developerboard" for DEVICE_TYPE which seems to have been invented when
these targets where added. The latter is not implemented in target.mk,
though, and will fall back to the "basic" set of packages then.

So, to clean this up and make it more readable, let's just define a
DEVICE_TYPE "basic" and use it for the aforementioned cases.

Cc: Christian Lamparter <chunkeey@gmail.com>
Cc: Sungbo Eo <mans0n@gorani.run>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>