REDHAT: Makefile, dont reset dist-git-tmp if set

JIRA: internal

This reverts my previous attempt at fixing this, and introduces a new fix.
Makefile.rhpkg is called on each invocation of @$(MAKE), because of it, RHDISTGIT_TMP is being redefined to the default value after the first invocation. This fixes that by checking if the variable has already been set before redefining it.

Signed-off-by: Lucas Zampieri lzampier@redhat.com