stlink: Give the NRST check more time as otherwise some L1 discovery board stayed in the bootloader.
STM32F4: Recognize F401.
STM32F4: Add handling of second bank and dual boot devices.
f4discovery: Limit clock to 48 MHz and stack 24 kiByte so that code can be compiled unchanged for F401Discovery too.

TODO: Bump clk up to 84 MHz as soon as libopencm3 supports that speed.
Revert "STM32F4: Add handling of second bank and dual boot devices."

This reverts commit 536482f8047f1ee984f362c5c5e3fe488353a1b7.