Initial riscv64 support

Most of the changes are self-explanatory and just follow what's
already happening for existing architectures.

Two things are worth pointing out:

  * we need to use shim-unsigned instead of shim-signed because
    riscv64 is not fully integrated into Fedora yet and so we
    can't do Secure Boot signing for the time being;

  * we use ext4 as bootfilesystem since in most cases the
    underlying firmware is going to be U-Boot, which needs to be
    able to load the board's DTB from /boot and doesn't support
    XFS.

Signed-off-by: David Abdurachmanov <davidlt@rivosinc.com>

teams/cloud/cloud.xml

@@ -89,7 +89,7 @@
 			</oemconfig>
 		</type>
 	</preferences>
-	<preferences profiles="Cloud-Base-Generic" arch="x86_64,aarch64">
+	<preferences profiles="Cloud-Base-Generic" arch="x86_64,aarch64,riscv64">
 		<type image="oem" format="qcow2"
 		      filesystem="btrfs" btrfs_root_is_subvolume="true" btrfs_set_default_volume="false" fsmountoptions="compress=zstd:1"
 		      kernelcmdline="no_timer_check console=tty1 console=ttyS0,115200n8 systemd.firstboot=off"
@@ -149,7 +149,7 @@
 			</oemconfig>
 		</type>
 	</preferences>
-	<preferences profiles="Cloud-Base-UEFI-UKI" arch="x86_64,aarch64">
+	<preferences profiles="Cloud-Base-UEFI-UKI" arch="x86_64,aarch64,riscv64">
 		<type image="oem" format="qcow2"
 		      firmware="uefi" efipartsize="1000"
 		      bootpartition="false"