Collect common QEMU arguments into common place

Use a single variable for the common QEMU arguments and document how to
run U-Boot in QEMU.
Signed-off-by: Grant Likely <grant.likely@arm.com>

board-qemu-arm.mk

@@ -31,14 +31,18 @@ ifeq ($(CONFIG_OPTEE),y)
 endif
 	cp $(UBOOT_OUTPUT)/u-boot.bin output/bl33.bin
 
+# Core QEMU configuration.
+QEMU_BASE_CONFIG += -machine virt,secure=on -cpu cortex-a57
+QEMU_BASE_CONFIG += -smp 2 -m 1024 -d unimp -monitor null -no-acpi
+QEMU_BASE_CONFIG += -nographic
+QEMU_BASE_CONFIG += -serial stdio  # Non-secure; u-boot console
+QEMU_BASE_CONFIG += -serial tcp::5000,server,nowait # Secure; optee
+ifneq ($(VIRTDISK),)
+QEMU_BASE_CONFIG += -drive if=virtio,format=raw,file=$(VIRTDISK)
+endif
+
 qemu-fip:
-	qemu-system-aarch64 -nographic -machine virt,secure=on -cpu cortex-a57 \
-		-no-acpi -smp 2 -m 1024 -bios $(FLASH_IMAGE) -d unimp \
-		-monitor null -serial stdio -serial tcp::5000,server,nowait
+	qemu-system-aarch64 $(QEMU_BASE_CONFIG) -bios $(FLASH_IMAGE) $(QEMU_EXTRA)
 
 qemu-semihosting:
-	cd output && qemu-system-aarch64 -nographic -machine virt,secure=on -cpu cortex-a57 \
-		-no-acpi -smp 2 -m 1024 -bios bl1.bin -d unimp \
-		-monitor null -serial stdio -serial tcp::5000,server,nowait \
-		-semihosting-config enable,target=native
-
+	cd output && qemu-system-aarch64 $(QEMU_BASE_CONFIG) -bios bl1.bin -semihosting-config enable,target=native $(QEMU_EXTRA)

top.mk

@@ -60,6 +60,17 @@ help:
 	@echo  '  devicetree/<name>	  - Build Devicetree target <name> *'
 	@echo  '    * Can only be called if U-Boot is configured'
 	@echo  ''
+	@echo  'Emulation targets (if available for given u-boot configuration:'
+	@echo  ''
+	@echo  '  qemu-fip:		  - Boot in QEMU using FIP image as -bios'
+	@echo  '  qemu-semihosting:	  - Boot in QEMU with semihosting'
+	@echo  ''
+	@echo  '  QEMU configuration can be manipulated with environmental variables:'
+	@echo  '  VIRTDISK	Path to a raw disk image. Could be an ISO, or a GPT or MBR'
+	@echo  '		partitioned disk image. The image will be passed to QEMU as a'
+	@echo  '		virtio block device'
+	@echo  '  QEMU_EXTRA	List of extra command line options passed to QEMU'
+	@echo  ''
 	@echo  'Other targets:'
 	@echo  ''
 	@echo  '  help		  - This help text'