populate_sdk_ext.bbclass: fix for multilib

populate_sdk_ext.bbclass: fix for multilib

[YOCTO #14020]

Fixed:
MACHINE = "qemux86-64"
require conf/multilib.conf
MULTILIBS = "multilib:lib32"
DEFAULTTUNE_virtclass-multilib-lib32 = "x86"

$ bitbake <image> -cpopulate_sdk_ext
$ bitbake <image> -ctestsdkext
[snip]
Standard Output: /bin/sh: 1: i686-wrsmllib32-linux-gcc: not found
[snip]

It was failed because no lib32 toolchain or lib installed.

This patch fixes:
* Set SDK_TARGETS correctly
* Return multilib depends in get_ext_sdk_depends()
* Write information to all environment-setup-* scripts.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>