populate_sdk: enable basic multilib support

populate_sdk: enable basic multilib support

In order to enable basic multilib support for the export of an SDK
image, a number of minor changes had to be made:

The value of MULTIMACH_TARGET_SYS needs to be variable.  This way we
can define the value to the appropriate multilib.  (Also in some cases
the default PACKAGE_ARCH is set to MACHINE_ARCH which is incorrect for
the SDK.)  Add a companion REAL_MULTIMACH_TARGET_SYS, based on code
from meta-environment, to allow for this.

We have to convert the do_populate_sdk into a python call, and then
break up the previous items into three parts.
  * Image construction
  * Setup of environment files
  * Generation of the tarball

Then we can iterate over the multilibs to populate the environment files.

Finally, matching changes were needed in the toolchain-scripts file.  And
what I presume is an optimization of immediate evaluation for
TOOLCHAIN_CONFIGSITE_NOCACHE and ..._SYSROOTCACHE needed to be done at
runtime, otherwise the wrong values may be used.

(From OE-Core rev: 42545ffbb37f2646a2a8c20999c21d3300e24f59)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>