Makefile: Rename compile_ld() to compile_elf()

The compile_ld() is actually used to create ELF files so
the name is misleading hence this patch renames it to
compile_elf(). We also rename LDFLAGS to ELFFLAGS because
these will be used for ELF creation only.

Signed-off-by: Anup Patel <anup.patel@wdc.com>