roms/Makefile: Build the generic platform for RISC-V OpenSBI firmware

roms/Makefile: Build the generic platform for RISC-V OpenSBI firmware

The RISC-V generic platform is a flattened device tree (FDT) based
platform where all platform specific functionality is provided based
on FDT passed by previous booting stage. The support was added in
the upstream OpenSBI v0.8 release recently.

Update our Makefile to build the generic platform instead of building
virt and sifive_u separately for RISC-V OpenSBI firmware, and change
to use fw_dynamic type images as well.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1596439832-29238-4-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>