Nuvoton's sample startup_M251.S file handles enough initialization for
my purposes, so I can completely bypass _start and jump directly to
main. Note that I also had to add a define to enable clearing of BSS.
The default values for SystemCoreClock, CyclesPerUs, and PllClock work
fine for my purposes of running from the 48 MHz HIRC. Remove unnecessary
initialization code. This is especially useful for the bootloader where
flash space is at a premium.
Also strip out unneeded UART setup code.
This will be used during firmware updates so that the main firmware can
communicate to the bootloader that it should stay in the bootloader for
a firmware update rather than run the main firmware again.
