Various changes/fixes to get configs/stm32f4discovery/kostest working after the big configuration renaming (and after a long period of bit rot)

STM32 FLASH fixes: use size_t instead of uint16_t, make interface more generic.  From Freddie Chopin

Change CONFIG_ADDRENV to CONFIG_ARCH_ADDRENV; change how it is selected -- the architecure must first declare support

wdog.h does not contain any application interface, only internal OS interface.  Further, it is non-standard.  Move wdog.h from include/ to include/nuttx. For the same reason, move the description of the watchdog timer interfaces from the Users Guide to the Porting Guide.