x86: Fix CONFIG_X86_GRUB_SERIAL=""

With

    .config:CONFIG_X86_GRUB_SERIAL=""

which (AFAICT) is the way to tell GRUB not to use a serial console, in

    target/linux/x86/image/Makefile:ifneq ($(CONFIG_X86_GRUB_SERIAL),)

$(CONFIG_X86_GRUB_SERIAL) expands to `""' (a literal double double-quote),
making the condition unconditionally false.

This patch fixes the situation by passing CONFIG_X86_GRUB_SERIAL through
qstrip before testing.

Signed-off-by: Tamas TEVESZ <ice@extreme.hu>

git-svn-id: svn://svn.openwrt.org/openwrt@37749 3c298f89-4303-0410-b956-a3cf2f4a3e73