populate_sdk_base: Fix grep command usage on old hosts

"man grep" on centos:

       -R, -r, --recursive
              Read all files under each directory, recursively; this is equivalent to the -d recurse option.

"man grep" on a more recent ubuntu system:

       -r, --recursive
              Read all files under each directory, recursively, following symbolic links only if they are on the command line.  This
              is equivalent to the -d recurse option.

So we have an issue when the SDK installer (even with
buildtools-tarball) is used on old hosts since it may try and
dereference paths which it should not. This is caused by differences in
the behaviour of grep -r on older systems.

The fix is to wrap this in find so that only real files are found (as
elsewhere in the script.

[YOCTO #6577]

(From OE-Core rev: 7986adeac16550b33f65fded39a55f668e0e543f)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>