scripts/qemustart: Allow specifying custom kernel for x86

Currently it's not possible to test boot squashfs root images, so this
patch now allows this use case as well.

Cc: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

scripts/qemustart

@@ -233,9 +233,10 @@ start_qemu_malta() {
 }
 
 start_qemu_x86() {
+	local qemu_exe
+	local kernel="$o_kernel"
 	local rootfs="$o_rootfs"
 	local mach="${o_mach:-pc}"
-	local qemu_exe
 
 	[ -n "$rootfs" ] || {
 		rootfs="$o_bindir/openwrt-$o_target-${o_subtarget%-*}-combined-ext4.img"
@@ -257,6 +258,13 @@ start_qemu_x86() {
 			;;
 	esac
 
+	[ -n "$kernel" ] && {
+	    o_qemu_extra+=( \
+		"-kernel" "$kernel" \
+		"-append" "root=/dev/vda console=ttyS0 rootwait" \
+	    )
+	}
+
 	[ -n $o_nonetwork ] || {
 		o_qemu_extra+=( \
 			"-netdev" "bridge,id=lan,br=$BR_LAN,helper=$HELPER" \