ARM: zynq: fix environment command syntax

Update EXTRA_ENV_SETTINGS and related commands to use 'setenv'
instead of short name 'set' in commands.

E.g. in case command setexpr is enabled the short form does not work
properly as the name becomes ambigous.

Fixes error messages like:

    U-Boot> set
    Unknown command 'set' - try 'help'

Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>