populate_sdk_ext: consider custom configuration in local.conf

populate_sdk_ext: consider custom configuration in local.conf

Copy the contents of local.conf under TOPDIR into the final generated
local.conf. In this way, custom settings are also made into the final
local.conf like IMAGE_INSTALL, DISTRO_FEATURES, VIRTUAL-RUNTIME_xxx, etc.

Before this change, installing extensible SDK would usually report failure
when preparing the build system if the user has custom configuration for
DISTRO_FEATURES in local.conf. Also, items in IMAGE_INSTALL_append in local.conf
also don't get built correctly.

This patch solves the above problem by making use of the bb.utils.edit_metadata.

In addition, we check to avoid any setting that might lead to host path
bleeding into SDK's configuration. Basically, variables with values starting
with '/' are removed. A whitelist mechanism is introduced so that users could
specify variables that should not be ignored. The name of the whitelist is
SDK_LOCAL_CONF_WHITELIST.

[YOCTO #7616]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>