[image_generator] add *.mif memory initialization file format

[stnolting]

The image generator / application makefile can already generate standard FPGA memory initialization files (*.mem and *.coe as well as a plain hex dump file). This PR also adds support for the *.mif format.

Requested in #947 (comment) by @alexazon.

Example

$ make mif

Generates neorv32_raw_exe.mif:

DEPTH = 273;
WIDTH = 32;
ADDRESS_RADIX = HEX;
DATA_RADIX = HEX;
CONTENT
BEGIN
00000000 : 000020b7;
00000001 : 80008093;
00000002 : 30009073;
...
0000010d : 40a00533;
0000010e : f61ff0ef;
0000010f : 40b00533;
00000110 : 00028067;
END;