lib: Rename string.x to sbi_string.x

All string functions are part of libsbi. It makes more sense
to rename them to sbi_string.x as the libsbi can be linked
with external libraries that can have similar implementation.

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

lib/sbi/objects.mk

@@ -27,3 +27,4 @@ libsbi-objs-y += sbi_system.o
 libsbi-objs-y += sbi_timer.o
 libsbi-objs-y += sbi_tlb.o
 libsbi-objs-y += sbi_trap.o
+libsbi-objs-y += sbi_string.o