ARC: build: remove non-existing bootpImage from KBUILD_IMAGE

ARC: build: remove non-existing bootpImage from KBUILD_IMAGE

The deb-pkg builds for ARCH=arc fail.

  $ export CROSS_COMPILE=<your-arc-compiler-prefix>
  $ make -s ARCH=arc defconfig
  $ make ARCH=arc bindeb-pkg
  SORTTAB vmlinux
  SYSMAP  System.map
  MODPOST Module.symvers
  make KERNELRELEASE=5.10.0-rc4 ARCH=arc KBUILD_BUILD_VERSION=2 -f ./Makefile intdeb-pkg
  sh ./scripts/package/builddeb
  cp: cannot stat 'arch/arc/boot/bootpImage': No such file or directory
  make[4]: *** [scripts/Makefile.package:87: intdeb-pkg] Error 1
  make[3]: *** [Makefile:1527: intdeb-pkg] Error 2
  make[2]: *** [debian/rules:13: binary-arch] Error 2
  dpkg-buildpackage: error: debian/rules binary subprocess returned exit status 2
  make[1]: *** [scripts/Makefile.package:83: bindeb-pkg] Error 2
  make: *** [Makefile:1527: bindeb-pkg] Error 2

The reason is obvious; arch/arc/Makefile sets $(boot)/bootpImage as
the default image, but there is no rule to build it.

Remove the meaningless KBUILD_IMAGE assignment so it will fallback
to the default vmlinux. With this change, you can build the deb package.

I removed the 'bootpImage' target as well. At best, it provides
'make bootpImage' as an alias of 'make vmlinux', but I do not see
much sense in doing so.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>