bitbake.conf: Remove double slash from PATH_prepend and PKG_CONFIG_DIR

bitbake.conf: Remove double slash from PATH_prepend and PKG_CONFIG_DIR

* we correctly have
  ${STAGING_DIR_NATIVE}${base_sbindir_native}
  and then double slash in
  ${STAGING_DIR_NATIVE}/${base_bindir_native}
* similar in PKG_CONFIG_DIR where libdir also starts with slash
  ${STAGING_DIR_HOST}/${libdir}/pkgconfig
* also fix double slash in insane.bbclass and staging.bbclass
* I was a bit nervous about staging change (in case the / was important
  in some weird use-case, but the extra slash is there since following
  commit where other extra slashes were removed only the one before
  libdir was kept:

  commit 6ea78d648951e5bbe9669412c0863daaf7f49ca5
  Author: Richard Purdie <rpurdie@linux.intel.com>
  Date:   Mon Nov 2 17:10:51 2009 +0000

  autotools.bbclass: Separate out useful staging functions into
                     base.bbclass and call from autotools classes
* this isn't fixing any real-world issue AFAIK, I was just trying to
  debug one weird case where debugedit fails with
  canonicalization unexpectedly shrank by one character
  and it's easier to grep for '//' without many harmless instances
  already in run* scripts etc

(From OE-Core rev: 0ddaf52e9e344986ae2b016cc068d9eee71b4347)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>