selftests/nolibc: add a "kernel" target to build the kernel with the initramfs

The "kernel" target rebuilds the kernel with the current config for the
selected arch, with an initramfs containing the nolibc-test utility.

Since image names depend on the architecture, the currently supported
ones are referenced and resolved based on the architecture.
Signed-off-by: NWilly Tarreau <w@1wt.eu>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/nolibc/Makefile

@@ -12,6 +12,16 @@ include $(srctree)/scripts/subarch.include
 ARCH = $(SUBARCH)
 endif
 
+# kernel image names by architecture
+IMAGE_i386    = arch/x86/boot/bzImage
+IMAGE_x86     = arch/x86/boot/bzImage
+IMAGE_arm64   = arch/arm64/boot/Image
+IMAGE_arm     = arch/arm/boot/zImage
+IMAGE_mips    = vmlinuz
+IMAGE_riscv   = arch/riscv/boot/Image
+IMAGE         = $(IMAGE_$(ARCH))
+IMAGE_NAME    = $(notdir $(IMAGE))
+
 # OUTPUT is only set when run from the main makefile, otherwise
 # it defaults to this nolibc directory.
 OUTPUT ?= $(CURDIR)/
@@ -36,6 +46,9 @@ initramfs: nolibc-test
 	$(call QUIET_INSTALL, initramfs/init)
 	$(Q)cp nolibc-test initramfs/init
 
+kernel: initramfs
+	$(Q)$(MAKE) -C $(srctree) ARCH=$(ARCH) CC=$(CC) CROSS_COMPILE=$(CROSS_COMPILE) $(IMAGE_NAME) CONFIG_INITRAMFS_SOURCE=$(CURDIR)/initramfs
+
 clean:
 	$(call QUIET_CLEAN, nolibc-test)
 	$(Q)rm -f nolibc-test