feat(dhcp): boot RISC-V 64-bit UEFI clients with grub2

RISC-V 64-bit UEFI firmware identifies itself with DHCP option 93
client-system-architecture 27 (0x001b, IANA processor architecture
types). Neither DHCP backend knew the value: Kea handed such clients no
boot file and ISC dhcpd fell through to the /yaboot catch-all.

Add an xcat-riscv64 Kea client class and an ISC subnet branch that send
them boot/grub2/grub2.riscv64, the same shape as the aarch64 entries.
The UEFI HTTP boot id (0x001c) is left alone: it needs a URL boot file
and the HTTPClient vendor class, which is a separate change.

perl-xCAT/xCAT/DHCP/BootPolicy.pm

@@ -36,6 +36,11 @@ sub kea_client_classes {
             test             => 'option[93].hex == 0x000b',
             'boot-file-name' => 'boot/grub2/grub2.aarch64',
         },
+        {
+            name             => 'xcat-riscv64',
+            test             => 'option[93].hex == 0x001b',
+            'boot-file-name' => 'boot/grub2/grub2.riscv64',
+        },
         {
             name             => 'xcat-ppc64',
             test             => 'option[93].hex == 0x000c',

xCAT-server/lib/xcat/plugins/dhcp.pm

@@ -4467,6 +4467,9 @@ sub addnet
         push @netent,
           "    } else if option client-architecture = 00:0b { #aaarch64\n ";
         push @netent, "      filename \"boot/grub2/grub2.aarch64\";\n";
+        push @netent,
+          "    } else if option client-architecture = 00:1b { #riscv64 uefi\n ";
+        push @netent, "      filename \"boot/grub2/grub2.riscv64\";\n";
         push @netent,
           "    } else if option client-architecture = 00:0e { #OPAL-v3\n ";
         push @netent, "        option conf-file = \"http://$tftp$portsuffix/tftpboot/pxelinux.cfg/p/" . $net . "_" . $maskbits . "\";\n";