grub2: Support setting rootflags in config

The `AIMEEOS_DEFAULT_ROOTFLAGS` kconfig option can be used to set the
default value for the `rootflags` GRUB2 environment variable.  The value
of this variable will be passed along as command-line arguments to
whatever kernel is chosen at boot.

Since post-image scripts do not have access to kconfig option values, we
need to build the GRUB2 environment file in the
`GRUB2_INSTALL_IMAGES_CMDS` script fragment instead.

Config.in

@@ -30,3 +30,8 @@ config AIMEEOS_RPI
 	select BR2_TARGET_GRUB2_ARM_EFI
 	help
 	  Support running Aimee OS on Rasperry Pi devices
+
+config AIMEEOS_DEFAULT_ROOTFLAGS
+	string "Default kernel command line argumens"
+	help
+	  Additional command line arguments to pass to the kernel by default.