[U-Boot] U-boot UBI breaks build with -O0

Some of the debug functions for UBI are preprocessed out for U-boot (ifndef __UBOOT__), but their calls are optimized out at higher optimization levels, so this does not cause a problem. When building with -O0, the linker gives an error:

fs/ubifs/io.c:162: undefined reference to `dbg_leb_unmap'

This function is in a #ifndef __UBOOT__ section.

Kevin Smith