build: introduce ALT vendor/model/variant

build: introduce ALT vendor/model/variant

Some devices are produced and sold under different names. To debloat
the buildroot but keeping it complete, new variables are introduced to
handle different namings. Below an example taken from a recent PR[0]

    DEVICE_VENDOR := Arcadyan
    DEVICE_MODEL := ARV4520PW
    DEVICE_ALT0_VENDOR := Vodafone
    DEVICE_ALT0_MODEL := Easybox 800
    DEVICE_ALT1_VENDOR := Airties
    DEVICE_ALT1_MODEL := WAV-281

With this commit the buildroot is extended to take care of up to three
alternative namings. The primary title plus alternatives names (if
defined) are shown in the `make menuconfig` dialog. Selecting on of
devices automatically selects all alternative names as they share the
same profile.

A list of the newly introduced variables:

    DEVICE_ALT0_VENDOR :=
    DEVICE_ALT0_MODEL :=
    DEVICE_ALT0_VARIANT :=
    DEVICE_ALT1_VENDOR :=
    DEVICE_ALT1_MODEL :=
    DEVICE_ALT1_VARIANT :=
    DEVICE_ALT2_VENDOR :=
    DEVICE_ALT2_MODEL :=
    DEVICE_ALT2_VARIANT :=

[0]: https://github.com/openwrt/openwrt/pull/2229/files#diff-b436f01932a18876c27800ba183d95f6R140

Signed-off-by: Paul Spooren <mail@aparcar.org>