Add parameters to fill header version:
Two new options are added (m and n) to fill header version major and minor.
The default is v1.0 (major = 1, minor = 0)

Fix image header on big endian hosts:
Three header fields are not properly converted to little endian
before assignment, resulting in incorrect header while executing
stm32image on big endian hosts.

Convert the value of the header fields version_number,
image_checksum and edcsa_algorithm to little endian before the
assignment.

Don't force the base of strtol, since it's able to select the base
automatically depending on the prefix of the value.
This does not breaks the current build script that extracts the
addresses, including the 0x prefix, from the map file.
This change helps using stm32image in shell scripts where the
addresses can be computed using the shell arithmetic expansion
"$((...))", that produces a value in base decimal.

The variable stm32image_header is declared with global visibility
but is us...