qemu.inc: disable curl for qemu-nativesdk

qemu.inc: disable curl for qemu-nativesdk

We had added the --disable-curl to EXTRA_OECONF, but there is an
EXTRA_OECONF_virtclass-nativesdk += "foo", the
"EXTRA_OECONF_virtclass-nativesdk +=" equals to
"EXTRA_OECONF_virtclass-nativesdk =" (the "+" has no effect here), so we
should add the "--disable-curl" to EXTRA_OECONF_virtclass-nativesdk.

And change the "+=" to "=" to not confuse people.

[YOCTO #2305]

(From OE-Core rev: a69df1807af3e099685b766dac4d6c6919b01852)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>