lib: Include helper libc functions directly in libsbi.

libsbi needs some of the custom libc functions. It should be directly
included in libsbi instead of platform specific libraries.

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

lib/sbi/libc/objects.mk

@@ -0,0 +1,15 @@
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+# Copyright (c) 2019 Western Digital Corporation or its affiliates.
+#
+# Authors:
+#   Atish Patra<atish.patra@wdc.com>
+#
+
+libc_files = string.o
+
+$(foreach file, $(libc_files), \
+	$(eval CFLAGS_$(file) = -I$(src)/../../sbi/libc))
+
+libsbi-objs-y += $(addprefix libc/,$(libc_files))