In case both buildkernel.conf and genkernel.conf set the same variable (e.g. ADDITIONALKERNELCMDS) the one in genkernel.conf takes precedence and the one in buildkernel.conf will be silently ignored. So I wonder, why in the first place encourage users to configure these variables in buildkernel.conf and not in genkernel.conf?
i.e. man buildkernel.conf says
ADDITIONALKERNELCMDS
If you wish to pass any additional command line parameters in the kernel boot line (either for attention of the kernel itself, or for genkernel(8)'s
init(8) script), you can specify them here. All of the important values for booting will be set by buildkernel, so this defaults to an empty string if
not set.
One parameter you may wish to consider here is "root_trim=yes"; this will enable TRIM support on the encrypted LUKS partition (turned off by default).