build: Add QEMU extra flag to be defined in apps

We want to provide to Qemu extra flags such as extra '-serial'
parameter. If we extend vQEMU_FLAGS then serial device goes to wrong
UART since first '-serial ${iQEMU_CONSOLE}' is already defined. The
order of UARTs does matter.

Change-Id: I6cd9cc84898a858e73d13297c27eeeac28010d9e
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>

make/target/run/rules.qemu

@@ -41,7 +41,7 @@ include ${vMAKE}/target/run/rules.qemu.${vHOST}
 	$q${HOS_Echo} [rules.qemu] Booting $<
 	$q${HOS_Echo} [${vQEMU_BIN} ${vQEMU_FLAGS}]
 	$q${HOS_Echo} ${iQEMU_BANNER}
-	$q${vQEMU_BIN} ${vQEMU_FLAGS} -kernel $< -serial ${iQEMU_CONSOLE}
+	$q${vQEMU_BIN} ${vQEMU_FLAGS} -kernel $< -serial ${iQEMU_CONSOLE} ${QEMU_EXTRA_FLAGS}
 
 .PHONY: help_qemu
 iQEMU_RUN_STR = $(foreach exec,${iHELP_EXEC_NAMES},${exec}.qemu): run