busybox.inc: fix CONFIG_EXTRA_CFLAGS configmangle

busybox.inc: fix CONFIG_EXTRA_CFLAGS configmangle

With current busybox Kbuild, setting .config to:

  CONFIG_EXTRA_CFLAGS="foo" "bar"

and then running 'make oldconfig' results in .config containing:

  CONFIG_EXTRA_CFLAGS="foo"

ie the CONFIG_EXTRA_CFLAGS configmangle in the busybox.inc doesn't
currently work as intended. Remove the extra \" \" to ensure that
${HOST_CC_ARCH} gets added to CONFIG_EXTRA_CFLAGS.

(From OE-Core rev: 5f822373fadff2415cf1602b9d58fae0dbd33730)

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>