When modifying the Optional Data of a boot option with bcfg boot -opt
the result was corrupted data, for instance a concatenation of old data,
heap contents, and new data. This was due to a erronous calculation of
the original optional data length.
In addition to fixing the calculation, add explaining comments and
introduce a helper variable, to not abuse other variables and confuse
readers (including the author).
Signed-off-by: Tormod Volden <debian.tormod@gmail.com>
54469a6918