Use a DP3T switch to select between eMMC, USB (serial download), and uSD boot devices, corrected the SW_BACK and SW_HOME buttons to have 33Ohm pull-ups instead
of pull-downs since the respective boot config lines have pull-downs already set

These buttons should be untouched during boot otherwise the board will not boot properly (pressing SW_HOME during boot causes "SD power cycle enable/eMMC reset enable" and pressing SW_BACK causes the uSD to be treated as a 4-bit bus if it's the boot device otherwise it's a BOOTCONFIG[6:4]=011 "reserved" condition)