x86: Refactor u-boot.rom build rules

Refactor u-boot.rom build rules by utilizing quiet_cmd_ and cmd_
macros. Also make writing mrc.bin and pci option rom to u-boot.rom
optional and remove mrc.bin from its dependent file list as not
every x86 board port needs mrc binary blob.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>