[sparc64] linux-image: Install uncompressed kernel image

Requested by John Paul Adrian Glaubitz, with the explanation:

> GRUB doesn't really support compressed kernels with OpenFirmware, at
> least on SPARC.  It used to work with 2.02+patches but it doesn't
> work with GRUB 2.04~rc1 and upstream said that it's not really
> supported.

debian/changelog

@@ -1084,6 +1084,7 @@ linux (4.19.37-1) UNRELEASED; urgency=medium
   * udeb: Add all HWRNG drivers to kernel-image (see #923675)
   * lockdown: Refer to Debian wiki until manual page exists
   * [sparc64] linux-image: Recommend grub-ieee1275 instead of (removed) silo
+  * [sparc64] linux-image: Install uncompressed kernel image
   * [powerpc,ppc64,ppc64el] linux-image: Recommend grub-ieee1275
   * [i386] Add grub-efi-ia32 as an alternate recommended bootloader
   * linux-source: Recommend bison and flex, always needed to build the kernel

debian/config/sparc64/defines

@@ -5,13 +5,13 @@ flavours:
 kernel-arch: sparc
 
 [build]
-image-file: arch/sparc/boot/zImage
+image-file: vmlinux
 
 [image]
 configs:
 bootloaders: grub-ieee1275
 suggests: fdutils
-install-stem: vmlinuz
+install-stem: vmlinux
 
 [sparc64_description]
 hardware: uniprocessor 64-bit UltraSPARC