sys-kernel/coreos-modules: switch to gzip/zstd compression

With the default gzip compression the 60 MB limit for the vmlinuz
bundle of kernel+initramfs was reached. The limit comes from the size
of the /boot partition which is 128 MB large and the kernel needs to
fit twice, in addition to GRUB.

Use zstd for the initramfs as it provides a similar speed but better
compression. For the kernel we can't switch yet to zstd for arm64
but for amd64 it works.

sdk_container/src/third_party/coreos-overlay/sys-kernel/coreos-kernel/coreos-kernel-5.10.52.ebuild

@@ -11,6 +11,7 @@ KEYWORDS="amd64 arm64"
 RDEPEND="=sys-kernel/coreos-modules-${PVR}"
 DEPEND="${RDEPEND}
 	app-arch/gzip
+	app-arch/zstd
 	app-shells/bash
 	coreos-base/coreos-init:=
 	sys-apps/coreutils
@@ -65,6 +66,7 @@ src_prepare() {
 	# Symlink to bootengine.cpio so we can stick with relative paths in .config
 	ln -sv "${ROOT}"/usr/share/bootengine/bootengine.cpio build/ || die
 	config_update 'CONFIG_INITRAMFS_SOURCE="bootengine.cpio"'
+	config_update 'CONFIG_INITRAMFS_COMPRESSION_ZSTD=y'
 
 	# include all intel and amd microcode files, avoiding the signatures
 	local fw_dir="${ROOT}lib/firmware"

sdk_container/src/third_party/coreos-overlay/sys-kernel/coreos-modules/files/amd64_defconfig-5.10

@@ -76,6 +76,7 @@ CONFIG_ISCSI_IBFT=y
 CONFIG_ISCSI_IBFT_FIND=y
 CONFIG_ITCO_VENDOR_SUPPORT=y
 CONFIG_ITCO_WDT=m
+CONFIG_KERNEL_ZSTD=y
 CONFIG_KEXEC_FILE=y
 CONFIG_KPROBES_ON_FTRACE=y
 CONFIG_KVM=m