Separate out sec32init sections even when not doing code relocation.

The current code does not boot on QEMU with 256K roms when code
relocation is disabled, because QEMU only maps the last 128K of the
rom to low memory.  The seabios make_bios_writable() call copies the
full rom to low memory.

This patch separates out the init sections even if code relocations
are disabled.  This effectively ensures that make_bios_writable() code
is in the last 128K of the rom.

This also introduces a new build symbol (final_readonly_start) which
stores the address of where the final runtime read-only memory begins.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>