Makefile: Build ELF, BIN and LD script in platform build directory

Currently, the make rules for ELF, BIN and LD script are little
generic to allowing ELF, BIN and LD script to be anywhere in the
build directory. For OpenSBI firmwares, the ELF, BIN, and LD script
are always platform specific so we update make rules accordingly.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>