sdk/context.py: add ability to check for multilib version of target package

sdk/context.py: add ability to check for multilib version of target package

Add a named argument 'multilib' for the hasTargetPackage function. Its default
value is False. When setting to True, it will try to get the correct multilib
prefix from the sdk_env, the environment setup script.

We need this because we don't want unexpected run of some sdk test cases.
The following steps will generate error.

1. Enable multilib for qemux86-64
   require conf/multilib.conf
   MULTILIBS ?= "multilib:lib32"
   DEFAULTTUNE_virtclass-multilib-lib32 ?= "core2-32"
2. bitbake core-image-sato -c populate_sdk
3. bitbake core-image-sato -c testsdk

The error message is like below.

  No package 'gtk+-3.0' found
  RESULTS - buildgalculator.GalculatorTest.test_galculator - Testcase -1: FAILED

As we don't have lib32-gtk+3 installed, the test case should be skipped when
testing against the lib32 environment setup script.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>