* Enable the PARTITION_WALK property by default
Previously, the new PARTITION_WALK which searches for bootable
partitions after a failure had to be explicitly enabled. Change
the default to be enabled by default. It can be switched off by
setting PARTITION_WALK=0 in the EEPROM config.
* Optimise bootmain for size on Pi4
Pi4 only has a 512KB SPI flash EEPROM and the addition of features
plus fixes is now causing contention for space between the code and
the EEPROM config. Since bootmain is only responsible for loading
start.elf revert to the original configuration which is optimised
for size rather than speed. Pi5 continues to be optimised for speed.
