Unsetting is the same as not available. Having “=m” implies available after the file system is up. Having “=y” implies always available. Assuming the feature can be built as a module (and not all features can be modules), then you could simply build the module and copy the file in. Without either “=y” or “=m” you can’t use the feature at all.
On the system where you build the kernel install package “libncurses5-dev” (e.g., “sudo apt-get install libncurses5-dev”). This makes all of the text mode editors (e.g., “make O=/where/ever/top/level/output/goes/ nconfig”) available.
I copy my existing “/proc/config.gz” into the top level of the kernel output location (renamed as “.config”), and then run “make nconfig”. This sets the starting config to match your running system (aside from still needing CONFIG_LOCALVERSION set). From there you can use “make nconfig” to search for “dm_crypt” (see the search function hot key list at the bottom of the menu). If it allows using the “m” key to set up the feature, then the feature can be a module. If only the “y” key can be used, then you must rebuild all modules and the kernel tool (and in this case you want a new CONFIG_LOCALVERSION other than “-tegra” and a new population of modules at “/lib/modules/$(uname -r)/”.
Tip: You can directly edit “.config” for the CONFIG_LOCALVERSION="-tegra", or use the nconfig search feature and find “localversion”.