scriptutils: fix fetch_uri() to work with RSS

scriptutils: fix fetch_uri() to work with RSS

Since recipe-specific sysroots were implemented, devtool add and devtool
upgrade operations that fetch from a URL that requires native sysroot
dependencies will fail to work as there is no recipe-specific sysroot
set up for them during fetching. An example was any URL pointing to a
tarball compressed with xz, e.g. devtool upgrade on gnutls.

The most expedient way to fix this is to set up a dummy recipe-specific
sysroot to use for the fetch/unpack operations. We do this in the same
manner as bitbake -b does, so we're just taking all of the sysroot
components available and creating a sysroot from those rather than
ensuring the correct dependencies are there - this means that we're
still going to have problems if e.g. xz-native hasn't been built yet,
but that issue will be trickier to solve and is tracked separately.

Fixes [YOCTO #11474].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>