illumos gate

The gptzfsboot and isoboot build depend on exact ordering of [entry] function and files and we need to be careful not to include extra code we do not want/need in boot2.

Simplify our life by using linker script to state the entry symbol and exclude linker sets. Entry function needs to be stored as very first function and we do not provide additional interactive commands. Also our fake multiboot structure needs to be at the beginning of the binary.

While there, clean up the Makefiles about {} constructs and build with -ffunction-sections -fdata-sections to make --gc-sections option to have chance to work.