tools/7z: cleanup Makefile

tools/7z: cleanup Makefile

Define the version in one variable,
and use Make functions to use variations of it elsewhere.

Correct the CPE ID.

Override the default tar directory flag
by adding a new value to the tar options
instead of defining the entire command.

Use variables for adjusting build recipes
instead of adding custom build recipes.

Remove unnecessary lines and add spacing.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/15991
Signed-off-by: Robert Marko <robimarko@gmail.com>