powerpc: boot: fix build with parallel make

powerpc: boot: fix build with parallel make

The powerpc boot wrapper Makefile is not parallel build safe, causing fixdep
to fail reading dependency files of the addnote, hack-coff and mktree
utilities when concurrently building different image targets.

A typical failure looks like:

      Building modules, stage 2.
      HOSTCC  arch/powerpc/boot/addnote
      HOSTCC  arch/powerpc/boot/hack-coff
      DTC     arch/powerpc/boot/taishan.dtb
      HOSTCC  arch/powerpc/boot/addnote
      HOSTCC  arch/powerpc/boot/hack-coff
      MODPOST 800 modules
    fixdep: error opening depfile: arch/powerpc/boot/.hack-coff.d: No such file or directory
    scripts/Makefile.host:91: recipe for target 'arch/powerpc/boot/hack-coff' failed
    make[5]: *** [arch/powerpc/boot/hack-coff] Error 2
    make[5]: *** Waiting for unfinished jobs....
    fixdep: error opening depfile: arch/powerpc/boot/.addnote.d: No such file or directory
    scripts/Makefile.host:91: recipe for target 'arch/powerpc/boot/addnote' failed
    make[5]: *** [arch/powerpc/boot/addnote] Error 2
    rm arch/powerpc/boot/taishan.dtb
    arch/powerpc/Makefile:263: recipe for target 'cuImage.taishan' failed
    make[4]: *** [cuImage.taishan] Error 2
    make[4]: *** Waiting for unfinished jobs....

Add a GNU make specific .NOTPARALLEL pseudo rule to enforce sequential building
of the addnote, hack-coff and mktree executables.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>