tools: relocate-rela: Open binary u-boot file later

There is no value to open u-boot binary file so early. Better to check all
values first and then open binary file.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/9c2b4ebadbe83497db28af02f6af2623793ffdb6.1655299267.git.michal.simek@amd.com