fastboot: remove #ifdef CONFIG when it is possible

Much of the fastboot code predates the introduction of Kconfig and
has quite a few #ifdefs in it which is unnecessary now that we can use
IS_ENABLED() et al.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # on vim3l