#65 Improve const correctness

[Markus Elfring]

Would you like to apply the advices from the article
"http://en.wikipedia.org/wiki/Const_correctness" to
your sources?
I suggest to add the key word "const" to the type
specifiers for parameters like the following.

http://cvs.sourceforge.net/viewcvs.py/etherboot/etherboot/etherboot-5.5/src/include/etherboot.h?rev=1.17&view=markup
- rom, assigned_rom_segment (function "rom_address_ok")
- header (function "prepare_boot_params")