runqemu: add QB_ROOTFS_EXTRA_OPT parameter

Content of the optional parameter will be appended to the rootfs-device
in the qemu configuration, in case QB_ROOTFS_OPT is not specified.
By default this is empty.

Example use cases are:

Defining 'readonly' when using squashfs, so multiple instances of qemu
can share the same base image, something that cannot be done by just
specifying 'snapshot'.

Defining 'bootindex=0' which helps to get past the EFI shell in
ovmf-binary. This also enables the use case of running WIC images with
EFI bootloader through the testimage.bbclass.

(From OE-Core rev: e9b8c194636cb5505774a2a71bf54450580dd5b8)

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/qemuboot.bbclass

@@ -65,6 +65,10 @@
 #                   " -device virtio-serial-device -chardev socket,id=virtcon,port=@PORT@,host=127.0.0.1 -device virtconsole,chardev=virtcon"
 #                   Note, runqemu will replace "@PORT@" with the port number which is used.
 #
+# QB_ROOTFS_EXTRA_OPT: extra options to be appended to the rootfs device in case there is none specified by QB_ROOTFS_OPT.
+#                      Can be used to automatically determine the image from the other variables
+#                      but define things link 'bootindex' when booting from EFI or 'readonly' when using squashfs
+#                      without the need to specify a dedicated qemu configuration
 # Usage:
 # IMAGE_CLASSES += "qemuboot"
 # See "runqemu help" for more info
@@ -77,6 +81,7 @@ QB_OPT_APPEND ?= "-show-cursor"
 QB_NETWORK_DEVICE ?= "-device virtio-net-pci,netdev=net0,mac=@MAC@"
 QB_CMDLINE_IP_SLIRP ?= "ip=dhcp"
 QB_CMDLINE_IP_TAP ?= "ip=192.168.7.@CLIENT@::192.168.7.@GATEWAY@:255.255.255.0"
+QB_ROOTFS_EXTRA_OPT ?= ""
 
 # This should be kept align with ROOT_VM
 QB_DRIVE_TYPE ?= "/dev/sd"