linuxboot: remove curley brace expansion from u-root commands

Curley brace expansion is a bash-ism, so we can't use it for the
u-root command list.

This unfortunately also breaks the current Kconfig option since the
list needs to be separated by space instead of commas.

Change-Id: I429a52c1673e29b7180ee6f53deaa7a551a1a9b3
Signed-off-by: David Hendricks <dhendricks@fb.com>
Reviewed-on: https://review.coreboot.org/27967
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
3 files changed