[armel/rpi] Add flavour for Raspberry Pi and Raspberry Pi Zero

Nowadays, Raspberry Pi 2 and Rasberry Pi 3 works perfectly fine with
Debian (including the official kernel package or the userland). RPi 1
and RPi Zero have an SoC that contains an armv6-based CPU, this means
that it cannot work with an hardfloat ABI, that is armv7 based. So we
have to use the Debian armel userland for this reason. Both boards are
supported in the mainline linux kernel and not being supported in the
debian-kernel package is the only blocking point that prevent RPI 1 and
RPI Zero from being well supported in an official Debian distribution.
This commit add a new kernel flavour for enabling support for the both
platforms.