arch/x86: CONFIG_BOOTLOADER_UNKNOWN renamed to CONFIG_X86_MULTIBOOT

The only use of the BOOTLOADER_UNKNOWN config option is on x86, where
it controls whether a multiboot header is embedded in the output.
This patch renames the option to be more descriptive, and makes it
an x86-specific option, rather than a Zephyr top-level option.

This also enables X86_MULTIBOOT by default, since the header only
occupies 12-16 bytes of memory and is (almost always) harmless.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>