There's an item in "make menuconfig" where you can tell where to 
find the initramfs.

     There are two ways to build it: either provide a compressed archive 
or a list of files. I'm sure you can find many howtos on the web. I 
learned that ~10 years ago and built my own framework. Of course you 
need to put in some applications and the easiest is Busybox.

     This means you must build Busybox, preferably as a statically 
linked single executable. Building it statically is almost impossible if 
it is linked against glibc. To use another libc, I think the easiest way 
is to use the Buildroot toolchain; just configure Buildroot to build 
nothing but Busybox.

     Once you have Busybox, you only need to write shell scripts and 
discover the mysteries of starting up a Linux system. There's some fun...