I think that additional "boot-loader" is an Arch kernel thing. Some kind of hack that allows it to read an ini file for instructions how and from where to load the actual kernel. In any case it is not part of Sakaki's Gentoo releases.
Obviously the most straightforward method is to hack into the u-boot environment and change the boot sequence there. Since u-boot won't handle my SSD drive, I myself had no other choice than to pursue this way. It does raise something of an issue though, because for some reason changing the bootalt1 environment variable does not appear to "stick" the first time. It looks good, but after a power cycle characters are gone missing and the boot command fails. If I remember correctly I did manage to get it running once by prepending characters to the bootalt1 environment variable but I felt I could not trust this result and decided to build a serial cable (using the front edge connector, so no soldering to the board) after all and change the environment from there..
Edit: found it. Got my experiences mixed up with something someone else wrote on the subject before, but the above is accurate enough. In depth info in the link.
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