populate_sdk_base.bbclass: fix SDKTARGETSYSROOT value

populate_sdk_base.bbclass: fix SDKTARGETSYSROOT value

Currently, SDKTARGETSYSROOT points to PACKAGE_ARCH which, sometimes, can
be set to MACHINE_ARCH. When this happens, the default target sysroot
passed to the cross-canadian toolchain, which points to TUNE_PKGARCH,
will be different from the directory where the target sysroot has been
deployed.

In order to fix this, use REAL_MULTIMACH_TARGET_SYS variable instead of
MULTIMACH_TARGET_SYS.

[YOCTO #3784]

(From OE-Core rev: 41437aaac0cfc6f931d3b2974d380f20ec01f6e8)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>