arm64: configs: Add virtconfig

Provide a slimline configuration intended to be booted on virtual
machines, with the goal of providing a light configuration which will
boot on and enable features available in mach-virt.  This is defined in
terms of the standard defconfig, with an additional virt.config fragment
which disables options unneeded in a virtual configuration.

As a first step we just disable all the ARCH_ configuration options,
disabling the build of all the SoC specific drivers.  This results in a
kernel that builds about 25% faster in my testing, if this approach
works for people we can add further options.
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230203-arm64-defconfigs-v1-3-cd0694a05f13@kernel.orgSigned-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm64/Makefile

@@ -215,6 +215,12 @@ ifdef CONFIG_COMPAT_VDSO
 endif
 endif
 
+include $(srctree)/scripts/Makefile.defconf
+
+PHONY += virtconfig
+virtconfig:
+	$(call merge_into_defconfig_override,defconfig,virt)
+
 define archhelp
   echo  '* Image.gz      - Compressed kernel image (arch/$(ARCH)/boot/Image.gz)'
   echo  '  Image         - Uncompressed kernel image (arch/$(ARCH)/boot/Image)'

arch/arm64/configs/virt.config

+# CONFIG_ARCH_ACTIONS is not set
+# CONFIG_ARCH_SUNXI is not set
+# CONFIG_ARCH_ALPINE is not set
+# CONFIG_ARCH_APPLE is not set
+# CONFIG_ARCH_BCM is not set
+# CONFIG_ARCH_BCM2835 is not set
+# CONFIG_ARCH_BCMBCA is not set
+# CONFIG_ARCH_BCM_IPROC is not set
+# CONFIG_ARCH_BERLIN is not set
+# CONFIG_ARCH_BRCMSTB is not set
+# CONFIG_ARCH_EXYNOS is not set
+# CONFIG_ARCH_K3 is not set
+# CONFIG_ARCH_LAYERSCAPE is not set
+# CONFIG_ARCH_LG1K is not set
+# CONFIG_ARCH_HISI is not set
+# CONFIG_ARCH_KEEMBAY is not set
+# CONFIG_ARCH_MEDIATEK is not set
+# CONFIG_ARCH_MESON is not set
+# CONFIG_ARCH_MVEBU is not set
+# CONFIG_ARCH_NXP is not set
+# CONFIG_ARCH_MXC is not set
+# CONFIG_ARCH_NPCM is not set
+# CONFIG_ARCH_QCOM is not set
+# CONFIG_ARCH_RENESAS is not set
+# CONFIG_ARCH_ROCKCHIP is not set
+# CONFIG_ARCH_S32 is not set
+# CONFIG_ARCH_SEATTLE is not set
+# CONFIG_ARCH_INTEL_SOCFPGA is not set
+# CONFIG_ARCH_SYNQUACER is not set
+# CONFIG_ARCH_TEGRA is not set
+# CONFIG_ARCH_TESLA_FSD is not set
+# CONFIG_ARCH_SPRD is not set
+# CONFIG_ARCH_THUNDER is not set
+# CONFIG_ARCH_THUNDER2 is not set
+# CONFIG_ARCH_UNIPHIER is not set
+# CONFIG_ARCH_VEXPRESS is not set
+# CONFIG_ARCH_VISCONTI is not set
+# CONFIG_ARCH_XGENE is not set
+# CONFIG_ARCH_ZYNQMP is not set