build: introduce $(MKHASH)

Before this commit, it was assumed that mkhash is in the PATH. While
this was fine for the normal build workflow, this led to some issues if

    make TOPDIR="$(pwd)" -C "$pkgdir" compile

was called manually. In most of the cases, I just saw warnings like this:

    make: Entering directory '/home/.../package/gluon-status-page'
    bash: line 1: mkhash: command not found
    bash: line 1: mkhash: command not found
    bash: line 1: mkhash: command not found
    bash: line 1: mkhash: command not found
    bash: line 1: mkhash: command not found
    bash: line 1: mkhash: command not found
    bash: line 1: mkhash: command not found
    bash: line 1: mkhash: command not found
    [...]

While these were only warnings and the package still compiled sucessfully,
I also observed that some package even fail to build because of this.

After applying this commit, the variable $(MKHASH) is introduced. This
variable points to $(STAGING_DIR_HOST)/bin/mkhash, which is always the
correct path.

Signed-off-by: Leonardo Mörlein <me@irrelefant.net>