omap3: evm: Fixes for CONFIG_NAND, SPL_OS_BOOT, USB, and environment

- Pass MTDPARTS kernel arguments to kernel
- Use Kconfig CONFIG_NAND instead of CONFIG_SYS_EXTRA_OPTIONS="NAND"
- Call 'usb_stop' on kernel start
- Update Falcon mode setup to match other OMAP3 boards
- Use "uEnv.txt" as boot script instead of "boot.scr"