riscv: show help string for riscv-specific targets

Define the archhelp variable so that 'make ACRH=riscv help' will show
the targets specific to building a RISC-V kernel like other
architectures.
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20240504193446.196886-3-emil.renner.berthing@canonical.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Makefile

@@ -216,3 +216,20 @@ rv32_defconfig:
 PHONY += rv32_nommu_virt_defconfig
 rv32_nommu_virt_defconfig:
 	$(Q)$(MAKE) -f $(srctree)/Makefile nommu_virt_defconfig 32-bit.config
+
+define archhelp
+  echo  '  Image		- Uncompressed kernel image (arch/riscv/boot/Image)'
+  echo  '  Image.gz	- Compressed kernel image (arch/riscv/boot/Image.gz)'
+  echo  '  Image.bz2	- Compressed kernel image (arch/riscv/boot/Image.bz2)'
+  echo  '  Image.lz4	- Compressed kernel image (arch/riscv/boot/Image.lz4)'
+  echo  '  Image.lzma	- Compressed kernel image (arch/riscv/boot/Image.lzma)'
+  echo  '  Image.lzo	- Compressed kernel image (arch/riscv/boot/Image.lzo)'
+  echo  '  Image.zst	- Compressed kernel image (arch/riscv/boot/Image.zst)'
+  echo  '  vmlinuz.efi	- Compressed EFI kernel image (arch/riscv/boot/vmlinuz.efi)'
+  echo  '		  Default when CONFIG_EFI_ZBOOT=y'
+  echo  '  xipImage	- Execute-in-place kernel image (arch/riscv/boot/xipImage)'
+  echo  '		  Default when CONFIG_XIP_KERNEL=y'
+  echo  '  install	- Install kernel using (your) ~/bin/$(INSTALLKERNEL) or'
+  echo  '		  (distribution) /sbin/$(INSTALLKERNEL) or install to '
+  echo  '		  $$(INSTALL_PATH)'
+endef