This allows to boot a sequence of boot entries until one succeeds.

boot sources can be passed in $global.boot.default, which is now treated
as a list. Also a list of boot entries can be specified as arguments
to the boot script. The entries can be:

- a plain filename from /env/boot/
- a full path to an arbitrary file
- a directory containing boot entries

With this this command:

boot net nand-ubi /env/boot.d

would first use the /env/boot/net entry, if this fails the /env/boot/nand-ubi
entry and if this also fails the files from /env/boot.d/ (which could also
be links to boot scripts)

To make the above the default, global.boot.default would be specified as:

global.boot.default="net nand-ubi /env/boot.d"

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>