new commands: uuid and guid - generate random unique identifier

Those commands basis on implementation of random UUID generator version 4
which is described in RFC4122. The same algorithm is used for generation
both ids but string representation is different as below.

char:  0        9    14   19   24         36
       xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
UUID:     be     be   be   be       be
GUID:     le     le   le   be       be

Commands usage:
- uuid [<varname>]
- guid [<varname>]

The result is saved in environment as a "varname" variable if argument is given,
if not then it is printed.

New config:
- CONFIG_CMD_UUID

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: trini@ti.com