Users should be more specific regarding which image should be built as
'image-as-nokernel-initramfsrootfs', because otherwise this will also
apply to the (defaut) initramfs.

Otherwise the default initramfs would switch to uncompressed cpio,
which will become too large and be unbootable.

By limiting the scope of the additional class to specific images only,
this problem can be avoided. These specific images are meant to be used
by the buildscripts here, and are going to be recompressed, but the
default yocto builds otherwise won't.

Signed-off-by: André Draszik <andre.draszik@linaro.org>