tm32: stm32l15xxx_rcc: configure medium performance voltage range and zero wait-state when allowed by SYSCLK setting

Zero wait-state for flash can be configured when:
 Range 1 and SYSCLK <= 16 Mhz
 Range 2 and SYSCLK <= 8 Mhz
 Range 3 and SYSCLK <= 4.2 Mhz

Medium performance voltage range (1.5V) can be configured when SYSCLK is up to 16 Mhz and PLLVCO up to 48 Mhz.

Fixed gconfig target.

Approved-by: Gregory Nutt <gnutt@nuttx.org>

6loWPAN: Add option to dump buffers.  Fix some issues with calculation of the IEEE802.15.4 header size.

STM32:add I2C3 SDA pin mapping for STM32F411

Approved-by: Gregory Nutt <gnutt@nuttx.org>

wireless/ieee802154: Add initialization logic for loopback driver; configs/sim: Add configuration for testing 6loWPAN; net/sixlowpan: Fix for compilation with debug output enabled.