bootconfig: Fix negative seeks on 32-bit with LFS enabled

The upstream patch "tools/bootconfig: Cleanup bootconfig footer size calculations" caused build regressions on most 32-bit architectures.

That patch changes some expression types so that a negative offset to lseek() now becomes positive if LFS is enabled. We had to force LFS on for 32-bit architectures that we also switched to 64-bit time_t, hence 32-bit architectures were broken except for i386.