#2 Removed conv buffer in intelhex_readline()

[Robert James Kaes]

Added the ascii_hex_to_int() function to take a series
of ASCII characters and convert it into an integer.
This replaces the calls to strtoul(), which required
the setup up a temporary conversion buffer. Instead,
the ascii_hex_to_int() function does the conversion
inline. No copying required.

By removing the conversion setup buffer, it clarifies
and simplifies the intelhex_readline() function.

[Robert James Kaes]