The optional parameter bootable is added in gpt command to set the
partition attribute flag "Legacy BIOS bootable"

This flag is used in extlinux and so in with distro to select
the boot partition where is located the configuration file
(please check out doc/README.distro for details).

With this parameter, U-Boot can be used to create the boot partition
needed for device using distro.

example of use:

setenv partitions "name=u-boot,size=60MiB;name=boot,size=60Mib,bootable;\
                   name=rootfs,size=0"

> gpt write mmc 0 $partitions

> part list mmc 0

Partition Map for MMC device 0  --   Partition Type: EFI

Part	Start LBA	End LBA		Name
	Attributes
	Type GUID
	Partition GUID
  1	0x00000022	0x0001e021	"u-boot"
	attrs:	0x0000000000000000
	type:	ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
	guid:	cceb0b18-39cb-d547-9db7-03b405fa77d4
  2	0x0001e022	0x0003c021	"boot"
	attrs:	0x0000000000000004
	type:	ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
	guid:	d4981a2b-0478-544e-9607-7fd3c651068d
  3	0x0003c022	0x003a9fde	"rootfs"
	attrs:	0x0000000000000000
	type:	ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
	guid:	6d6c9a36-e919-264d-a9ee-bd00379686c7

> part list mmc 0 -bootable devplist

> printenv devplist

devplist=2

Then the distro scripts will search extlinux in partition 2
and not in the first partition.

Signed-off-by: Patrick Delaunay <patrick.delaunay73@gmail.com>