doc:bootmenu: add description for UEFI boot support

The bootmenu enumerates the UEFI boot options
for boot device selection.
This commit adds the description how the UEFI boot work
in bootmenu. This commit also adds "Synopsis", "Description"
and "Configuration" sections to follow the U-Boot command
documentation format.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>