main: include .data section in .hex file

The function extracting the firmware image for .hex and .bin files
wasn't working correctly: it only extracted the .text segment and not
the .data segment.
This commit fixes this issue, so that it behaves (hopefully) just like
objcopy -O{ihex|binary}.

Another small change is that the formatting of the .hex file was made
more like the output of objcopy: no entry addres (old Intel CPU
holdover) and 16 bytes of data on each line.

uf2.go

@@ -15,7 +15,7 @@ import (
 // ConvertELFFileToUF2File converts an ELF file to a UF2 file.
 func ConvertELFFileToUF2File(infile, outfile string) error {
 	// Read the .text segment.
-	_, data, err := ExtractTextSegment(infile)
+	_, data, err := ExtractROM(infile)
 	if err != nil {
 		return err
 	}