Environment handling fixes and support for multiple environment backends

[Luotao Fu]

Hi,
the following patch set fixes and changes the environment handling in barebox.
The main change is that we have now support for multiple environment backends in
backup. This option is configurable. Once activated, barebox will scan for all
environment backends during startup, load the environment from the first loadble
backend and than synchronize the content of all remaining (or previous)
backends. The commands savenenv and loadenv are also extended with the support
for this option.

diffstat:

commands/loadenv.c    |   51 +++++++++++++++++++++-----
commands/saveenv.c    |   94 ++++++++++++++++++++++++++---------------------
common/Kconfig        |   11 ++++++
common/environment.c  |   53 +++++++++++++++++++++++++--
common/startup.c      |   97 ++++++++++++++++++++++++++++++++++++++++++++-----
include/environment.h |    3 +-
scripts/bareboxenv.c  |    2 +-
7 files changed, 246 insertions(+), 65 deletions(-)