btrfs-progs: build: allow '--with-convert=' to accept comma-separated values

btrfs-progs: build: allow '--with-convert=' to accept comma-separated values

Before the change configure refused to accept it's defaults explicitly:
    $ ./configure --enable-convert --with-convert=ext2,reiserfs
    ...
    configure: error: unknown tokens for --with-convert: ,

After the change both converters are enabled:
    $ ./configure --enable-convert --with-convert=ext2,reiserfs
    ...
            btrfs-convert:      yes (ext2,reiserfs)

Cc: linux-btrfs@vger.kernel.org
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: David Sterba <dsterba@suse.com>