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>

uki-editbootconfig.sh

@@ -3,8 +3,9 @@ echo "###" "$0" "$@"
 
 # set arch-specific variables
 case "$(uname -m)" in
-    aarch64) arch="aa64"; ARCH="AA64";;
-    x86_64)  arch="x64";  ARCH="X64";;
+    aarch64) arch="aa64";    ARCH="AA64";;
+    riscv64) arch="riscv64"; ARCH="RISCV64";;
+    x86_64)  arch="x64";     ARCH="X64";;
 esac
 
 # figure where shim.efi and BOOT.CSV are located