binman: Overwrite symlink if it already exists

Without this re-building will fail with an error when trying to create
the symlink for the second time with an already exists error.

Signed-off-by: Andrew Davis <afd@ti.com>
[n-francis@ti.com: Added support for test output dir and testcase]
Signed-off-by: Neha Malcom Francis <n-francis@ti.com>