top: Improve inst_file_list() in Makefile for payloads installation

We now have payloads sub-directory under firmware directory which
means payload BINs and ELFs should be installed under firmware/payloads.

This patch improves inst_file_list() to handle payloads sub-directory
under firmware installation directory.

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