Squashed commit of the following:

Author: Juha Niskanen <juha.niskanen@haltian.com>
Date:   Mon Feb 19 15:03:17 2018 -0600

    drivers/mtd:  mtd_config.c:  Add still more error handling (to detect bad underlying flash implementations)
    drivers/mtd:  mtd_config.c:  Remove MTD_ERASE that was erasing data block instead of erase block.  This is a partial revert of 4f18b4.  Reported-by: Pascal Speck <iktek01@yahoo.com>
    arch/arm/src/stm32l4:  stm32l4_flash: change flash programming to use page buffer for unaligned writes.