m/n/b/mkpayload: support multiple initramfs files

Linux supports multiple concatinated initramfs files and requires using
multiple to provide certain data like microcode for the early loader.
This allows building such payloads with mkpayload.

Change-Id: Ie7ee7886bbfe481d7b723e0476a26ee26425a0b6
Reviewed-on: https://review.monogon.dev/c/monogon/+/548
Reviewed-by: Mateusz Zalega <mateusz@monogon.tech>
diff --git a/metropolis/installer/BUILD.bazel b/metropolis/installer/BUILD.bazel
index 834dd9a..61e455e 100644
--- a/metropolis/installer/BUILD.bazel
+++ b/metropolis/installer/BUILD.bazel
@@ -40,7 +40,7 @@
 efi_unified_kernel_image(
     name = "kernel",
     cmdline = "console=ttyS0,115200 console=tty0 quiet",
-    initramfs = ":initramfs",
+    initrd = [":initramfs"],
     kernel = "//third_party/linux",
     os_release = ":installer-release-info",
     visibility = ["//visibility:public"],
diff --git a/metropolis/installer/test/BUILD.bazel b/metropolis/installer/test/BUILD.bazel
index 59c5803..c958ac2 100644
--- a/metropolis/installer/test/BUILD.bazel
+++ b/metropolis/installer/test/BUILD.bazel
@@ -34,7 +34,7 @@
 efi_unified_kernel_image(
     name = "kernel",
     cmdline = "loglevel=0 console=ttyS0",
-    initramfs = "//metropolis/installer:initramfs",
+    initrd = ["//metropolis/installer:initramfs"],
     kernel = "//third_party/linux",
     visibility = ["//visibility:private"],
 )