packed_struct replaced by begin_packed_struct and end_packed_struct

NxWM configurations.  If using a 7-bit character set, then the cursor character cannot be 137 (graphic block).  Use 95 (underscore) instead.

STM32F428i-DISCO:  Change NxWM cursor character from 137 (graphics block) to 95 (underscore).  NxWM is configured to use a 7-bit character set so 137 is not a valid character code.

SAMV7 Ethernet: Fix a compiler error introduced with commit 7467329a

Add debug assertion in libdtoa to catch attempts to use floating point output formats from within an interrupt handler.  That will cause assertions or crashes downstream because __dtoa will attempt to allocate memory.