[coreboot] [PATCH]Flexible filename prefix

[Patrick Georgi]

Hi,

attached file is the first part of my quest for a fallback/normal switch
in the Kconfig regime.
It allows building images with different prefixes (ie. normal/romstage,
helloWorld/romstage, ...).
It defaults to fallback/, so there's no user visible change now.

It's intended to ultimately support switching between images in the
Tinybootblock build model, by running the build several times, adding to
the same image: First fallback, then normal, ...

Signed-off-by: Patrick Georgi