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>

grub-riscv64.cfg.iso-template

@@ -0,0 +1,41 @@
+# Inspired by the config used for lorax-built live media
+
+set default="1"
+
+function load_video {
+	insmod efi_gop
+	insmod efi_uga
+	insmod video_bochs
+	insmod video_cirrus
+	insmod all_video
+}
+set basicgfx="nomodeset"
+
+load_video
+set gfxpayload=keep
+insmod gzio
+insmod part_gpt
+insmod ext2
+
+terminal_input console
+terminal_output ${terminal_output}
+
+set timeout=${boot_timeout}
+set timeout_style=${boot_timeout_style}
+
+search ${search_params}
+
+menuentry "Start ${title}" --class fedora --class gnu-linux --class gnu --class os {
+	linux ($$root)${bootpath}/${kernel_file} ${boot_options}
+	initrd ($$root)${bootpath}/${initrd_file}
+}
+menuentry "Test this media & start ${title}" --class fedora --class gnu-linux --class gnu --class os {
+	linux ($$root)${bootpath}/${kernel_file} ${boot_options} rd.live.check
+	initrd ($$root)${bootpath}/${initrd_file}
+}
+submenu "Troubleshooting -->" {
+	menuentry "Start ${title} in basic graphics mode" --class fedora --class gnu-linux --class gnu --class os {
+		linux ($$root)${bootpath}/${kernel_file} ${boot_options} $${basicgfx}
+		initrd ($$root)${bootpath}/${initrd_file}
+	}
+}