populate_sdk_ext.bbclass: Be more permissive on the name of the buildtools

populate_sdk_ext.bbclass: Be more permissive on the name of the buildtools

We want to support different names for the buildtools tarball.  The
name may not always be of the default oe-core format.

For instance, at Wind River we define the built-tools name to be:

${SDK_ARCH}-buildtools-nativesdk-standalone-${DISTRO_VERSION}

because thes standard SDK_NAME has additional information that is not
relevant to the builtools tarball.

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>