tools/firmware-utils: mktplinkfw move build_fw() to lib

This patch moves build_fw() to mktplinkfw-lib.c

The versions of mktplinkfw.c and mktplinkfw2.c had slight
differences in code flow, the version from mktplinkfw.c has been
preferred.

While it's expected that this change will not affect mktplinkfw2,
all use cases could not be tested and so this particular change
is committed separately.

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>