Now also print warnings if there are missing features or a blacklisted
board when doing 'make flash'

Before, only when doing 'make' or 'make all' the FEATURES_REQUIRED,
BOARD_BLACKLIST/WHITELIST and TOOLCHAINS were checked and a warning was printed.

However as 'flash' triggers 'all' it is a common case to do 'make flash'
directly instead of 'make all flash'. So better also print warnings in
this case.