Improve input parsing of icebram
Improve input parsing of icebram

Add support for the following in the input hexfiles:

 * Horizontal whitespace (space, tab, cr)
 * Multiple words on the same line
 * Empty lines
 * Embedded underscores (_) in the words
 * x and z nibbles (interpreted as zero)

In addition, allow for the to_hexfile to be shorter than the
from_hexfile, padding with zeroes as needed.

SymbiFlow/icestorm,SymbiFlow/icestorm,SymbiFlow/icestorm,SymbiFlow/icestorm