Adapt to new U-Boot build commands

U-Boot commit 51148790f26e "kconfig: switch to Kconfig" changed the
commands required to configure U-Boot. Adapt the flasher build script
to those changes, in a way that automatically works with either old or
new U-Boot source trees.

Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
--
v2: Check for the modified "make help" output rather than a file added
by commit 51148790f26e. Hopefully this will be more independant from
changes to U-Boot's implementation details.