i guess you can do it either way a) pcmcia/ide support in kernel and use that (big!) kernel to boot from that device b) use ide<cf> adapater and smaller kernel with ide support only.
I don't think in-kernel will be enough. pcmica services depend on the card manager deamon to detect device insertions and register the device. cardmgr is user space.
it was my understanding that the reason the pcmcia/cardbus stuff got integrated into kernel in the first place was to support booting from such devices.
So I think you will have to go early userspace or an initrd and either pivot root or kexec into another kernel.
When I did my wireless pcmcia net boot setup I had to compile the pcmcia-cs stuff against uclibc to get it small and remove the glib dependency. I was placeing it all in 1Meg of ROM so I even went further and hacked down cardmgr so that all it would do was register the device. Then I built a small initrd with all the tools. The boot system then pulled the real kernel down off the network and kexeced into it.