[XTENSA] adjust boot linker script start addresses

Move boot-redboot load address from 0xD0200000 to 0xD1000000
to make space for larger kernel images, in particular those with
an embedded initramfs filesystem.
Also properly set the ELF start address in boot-elf images so
that PC need not be set manually when loading them using GDB.

Signed-off-by: Marc Gauthier <marc@tensilica.com>