x86: ifdtool: Split microcode linking into its own function

The code to set up the microcode pointer in the ROM shares almost nothing
with the write_uboot() function.

Move it into its own function so it will be easier to extend.

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