Add the manifest file for better compatibility with the existing SDE tools

[vgurevich]

A number of tools, developed for the original (Barefoot/Intel) SDE check for the .manifest file in $SDE directory. They do normally expect to find a file with the name bf-sde-<version_number>.manifest in $SDE. They often extract the "version_number" for their purposes.

Some tools might go further and might check the contents of the file for the list of packages and their versions that looks like this:

package1 : version1
package2 : version2
...
packageN : versionN

For those packages they would typically expect tarball in the directory $SDE/packages/ with the name $SDE/packages/package1-version1.tgz (or any other typical extension that is acceptable to tar -xf). The unpacking tools would be expected to extract such a tarball into $SDE/pkgsrc/package1 (i.e. the version number will be stripped).

For now, I propose that we simply add an empty file named bf-sde-open-p4studio.manifest into the top-level directory. It is not a panacea (e.g. some tools do explicitly check that the version_name has a special format and probably need to be changed in the due course), but it will help to revive the majority of the tools.