build: create tmp/userids file

build: create tmp/userids file

Multiple packages contain a USERID variable defining required user and
group for the package to run. With the recent addition of
"PKG_FILE_MODES" it is possible to define user and group of specific
files, replacing (possibly insecure) post-inst scripts. These modes are
set during build time and put directly into the packages.

To allow user and group names rather than the numeric values, a mapping
like `/etc/passwd` is required by the `ipkg-build` script, mapping names
defined in "PKG_FILE_MODES" to a numeric value, as the build system does
not create any users during build.

This commit adds a single line to the `prepare-tmpinfo` target, so that
everytime the feeds are updated the *passwd like* content of
`./tmp/userids` is updated.

Signed-off-by: Paul Spooren <mail@aparcar.org>