STM32F4 8-bit support for 1.8v operation

Per ST doc DM00035129.pdf "DocID022063 Rev 5", the STM32F415xx data sheet,
table 40 on page 110 of the PDF, 32-bit program operation is only possible
above 2.7 Volts.

In order to support programming on devices running at lower voltages, this
commit adds an 8-bit programming mode and the necessary tests to enable it
when the reported voltage is below 2.7 Volts.