build, imagebuilder: Do not require compilers

build, imagebuilder: Do not require compilers

The buildroot and SDK both require the compilers (gcc, g++) to be
installed on the host system, however the ImageBuilder uses precompiled
binaries.

This patch changes the prerequirements checks to skip the checking for
the compilers if running as ImageBuilder. A similar change has been
made for libncurses-dev in 4a1a58a3e2d2.

Signed-off-by: Sven Roederer <devel-sven@geroedel.de>
Acked-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit ae12a747cae3df16d84b7dc92f39427948d4e8e2)