poky-init-build-env: Enable all POSIX shells

Update the script so that it will work in more then just bash.  If bash
is not used, it will assume the PWD is the Poky location.  (This is because
BASH_SOURCE is a bash-ism, and equivalent functionality is not available in
other shells).

This has been verified with dash (see comment in the code), ksh, zsh and
of course bash.

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>

scripts/poky-env-internal

@@ -31,7 +31,7 @@ if [ "x$BDIR" = "x" ]; then
         BDIR=`readlink -f "$1"`
     fi
 fi
-if [[ "$BDIR" = /* ]] ; then
+if expr "$BDIR" : '/.*' > /dev/null ; then
     BUILDDIR="$BDIR"
 else
     BUILDDIR="`pwd`/$BDIR"