populate_sdk_ext.bbclass: use weak assignment for TOOLCHAINEXT_OUTPUTNAME

populate_sdk_ext.bbclass: use weak assignment for TOOLCHAINEXT_OUTPUTNAME

The TOOLCHAINEXT_OUTPUTNAME is different from TOOLCHAIN_OUTPUTNAME, it
is used for eSDK only, so that it doesn't mix with SDK, use "?=" for it
so that other conf file can define it.

If we don't use "?=" here, then we need use forcevariable to redfine it:
TOOLCHAINEXT_OUTPUTNAME_forcevariable = "foo"

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>