bugfix: stack corruption loading IHex images

The Hex parser uses a fixed number of sections.  When the
number of sections in the file is greater than that, the
stack get corrupted and a CHECKSUM ERROR is detected
which is very confusing.

This checks the number of sections read, and increases
IMAGE_MAX_SECTIONS so it works on my file.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>