ar71xx: fix giving extra arguments to Build/mktplinkfw

The build command will always get the whole argument string in $(1),
regardless of whitespace. We need to use word/wordlist to split the string
after the first word.

Whitespace and quotation will be given to the command verbatim (make will
ignore it), so to give multiple arguments, no quotation marks may be used.

Fixes: r47174 ("ar71xx/image: add options argument to mktplinkfw step")
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49216 3c298f89-4303-0410-b956-a3cf2f4a3e73