image.bbclass, populate_sdk_base.bbclass: make all variants of qemuwrapper-cross...

image.bbclass, populate_sdk_base.bbclass: make all variants of qemuwrapper-cross available in sysroots

The variants are needed in particular when executing postinst_intercepts as
those may require running binaries built for different architectures and
against different sets of library paths, when multilib is in use (or nativesdk host
packages are installed), so a single global variant of the script was not working.

I do understand expanding PATH and DEPENDS in this manner is hackish, however
every other approach I could think of is worse.

(From OE-Core rev: 2f31eecc40ea4d0865aa28d65a0ba7d5a629393a)

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>