CONFIG_KERNEL_XZ obscure build failure if xz(1) not installed

[Andy Isaacson <adi@hexapodia.org>]

% uname -m
x86_64
% git log -1 | head
commit 100b33c8bd8a3235fd0b7948338d6cbb3db3c63d
% grep XZ .config
CONFIG_HAVE_KERNEL_XZ=y
CONFIG_KERNEL_XZ=y
CONFIG_RD_XZ=y
CONFIG_SQUASHFS_XZ=y
CONFIG_XZ_DEC=y
CONFIG_XZ_DEC_X86=y
CONFIG_XZ_DEC_POWERPC=y
CONFIG_XZ_DEC_IA64=y
CONFIG_XZ_DEC_ARM=y
CONFIG_XZ_DEC_ARMTHUMB=y
CONFIG_XZ_DEC_SPARC=y
CONFIG_XZ_DEC_BCJ=y
CONFIG_XZ_DEC_TEST=y
CONFIG_DECOMPRESS_XZ=y
% make -j4
...
  CC      arch/x86/boot/tty.o
  CC      arch/x86/oprofile/oprofile.mod.o
  XZKERN  arch/x86/boot/compressed/vmlinux.bin.xz
exec: 23: xz: not found
  CC      arch/x86/boot/video.o
  CC      crypto/crc32c.mod.o
  CC      crypto/pcrypt.mod.o
  CC      arch/x86/boot/video-mode.o
  CC      drivers/acpi/acpi_memhotplug.mod.o
  MKPIGGY arch/x86/boot/compressed/piggy.S
arch/x86/boot/compressed/vmlinux.bin.xz: No such file or directory
  CC      arch/x86/boot/version.o
  LD      arch/x86/boot/compressed/vmlinux
... [hundreds of lines of output snipped]
  LD [M]  net/sched/act_csum.ko
  IHEX    firmware/bnx2/bnx2-mips-09-6.2.1.fw
  IHEX    firmware/bnx2/bnx2-mips-06-6.2.1.fw
% echo $?
2
% ls -l arch/x86/boot/bzImage
-rw-r--r-- 1 adi adi 1991504 Feb  1 18:41 arch/x86/boot/bzImage

The failure is obscured by trailing make output, and since a broken
bzImage is created, "make install" succeeds, and the first visible
failure is after reboot when I get the lovely

    Decompressing Linux...

    Input is not in the XZ format (wrong magic bytes)

     -- System halted

Sometimes I run "make -j4 && make install" which would have caught this,
but the error should be more prominent and we shouldn't create bzImage
if the dependencies are not present.

-andy