xtensa: nommu: fix Image.elf reset code and ld script

Don't hardcode kernel entry address as 0x3000 or 0xd0003000, use
LOAD_MEMORY_ADDRESS macro. Don't compile MMU remapping code and don't try
to link it when building noMMU configuration.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>