tools: add a simple script to generate EFI variables

This script generates EFI variables for U-Boot variable store format.

A few examples:

  - Generating secure boot keys

    $ openssl req -x509 -sha256 -newkey rsa:2048 -subj /CN=TEST_PK/ \
            -keyout PK.key -out PK.crt -nodes -days 365
    $ efisiglist -a -c PK.crt -o foo.esl
    $ tools/efivar.py set -i ubootefi.var -n db -d foo.esl -t file
    $ tools/efivar.py set -i ubootefi.var -n kek -d foo.esl -t file
    $ tools/efivar.py set -i ubootefi.var -n pk -d foo.esl -t file

  - Printing out variables

    $ tools/efivar.py set -i ubootefi.var -n var1 -d foo -t str
    $ tools/efivar.py set -i ubootefi.var -n var2 -d bar -t str
    $ tools/efivar.py print -i ubootefi.var
    var1:
        8be4df61-93ca-11d2-aa0d-00e098032b8c EFI_GLOBAL_VARIABLE_GUID
        NV|BS|RT, DataSize = 0x3
        0000000000: 66 6F 6F                                          foo
    var2:
        8be4df61-93ca-11d2-aa0d-00e098032b8c EFI_GLOBAL_VARIABLE_GUID
        NV|BS|RT, DataSize = 0x3
        0000000000: 62 61 72                                          bar

    - Removing variables

      $ tools/efivar.py del -i ubootefi.var -n var1
      $ tools/efivar.py set -i ubootefi.var -n var1 -a nv,bs -d foo -t str
      $ tools/efivar.py print -i ubootefi.var -n var1
      var1:
          8be4df61-93ca-11d2-aa0d-00e098032b8c EFI_GLOBAL_VARIABLE_GUID
          NV|BS, DataSize = 0x3
          0000000000: 66 6F 6F                                        foo
      $ tools/efivar.py del -i ubootefi.var -n var1
      err: attributes don't match
      $ tools/efivar.py del -i ubootefi.var -n var1 -a nv,bs
      $ tools/efivar.py print -i ubootefi.var -n var1
      err: variable not found

Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>

Correct examples in commit message.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>