MIPS: boot: merge build rules of vmlinux.*.itb by using pattern rule

Merge the build rule of vmlinux.{gz,bz2,lzma,lzo}.itb, and also move
'targets' close to the related code.

[paul.burton@mips.com:
  - Remove leading tabs from assignments to itb_addr_cells, since after
    this patch moves the additions to the 'targets' variable the
    assignments to itb_addr_cells wound up being treated as part of the
    uImage rule above them, causing the .its to incorrectly be generated
    with empty ADDR_CELLS.]

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Patchwork: https://patchwork.linux-mips.org/patch/19095/


Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org