Forcing the use of "\n" in mirror variables is pointless, we can just require that
there are pairs of values. With the bitbake restriction relaxed, we can relax
the sanity check too.
(From OE-Core rev: 7313b10e242da9225211ca9fd53d14a121c5fa42)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>