cmd: env: extend "env [set|print] -e" to manage UEFI variables

With this patch, when setting UEFI variable with "env set -e" command,
we will be able to
- specify vendor guid with "-guid guid",
- specify variable attributes,  BOOTSERVICE_ACCESS, RUNTIME_ACCESS,
  respectively with "-bs" and "-rt",
- append a value instead of overwriting with "-a",
- use memory as variable's value instead of explicit values given
  at the command line with "-i address,size"

If guid is not explicitly given, default value will be used.

Meanwhile, "env print -e," will be modified so that it will NOT dump
a variable's value if '-n' is specified.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>