stand/uboot: Remove -fPIC here

When needed, -fPIC is added in defs.mk. While not in main, mips on
stable/13 can't tolerate it. Remove it here.

MFC After: now (it's a build issue)
Sponsored by: Netflix

(cherry picked from commit 4410e857965edef0e3be42ae323356ec08d2aaad)