Add configurable boot config

[tasken]

Adds /_pico/boot.cfg for selecting an NDS file when the console starts.

• Map A, B, X, Y, SELECT, START, D-pad, L, and R to absolute SD-card paths.
• Use DEFAULT= when no mapped button is held. Return-capable apps launched directly also return to this path.
• Keep /_picoboot.nds as the fallback for missing, invalid, or unreadable entries.
• Hold L+R+SELECT at startup to ignore the configuration and boot /_picoboot.nds.
• Add a commented template at sdcard/_pico/boot.cfg.