cmd: pxe: add support for FIT config selection

cmd: pxe: add support for FIT config selection

Add a way in configuration files (exlinux.conf for sysboot command)
to select a specific FIT configuration. The configuration is selected
with a string added after the FIT filename in the label "KERNEL" or
"LINUX", using the same format than bootm command:

KERNEL [Filename]#<conf>[#<extra-conf[#...]]

This configuration string, beginning by '#', is directly appended
to bootm argument 1 after <kernel_addr_r>.

bootm [<kernel_addr_r>]#<conf>[#<extra-conf[#...]]

see doc/uImage.FIT/command_syntax_extensions.txt for details

Example :
 KERNEL /fit.itb#cfg1
 KERNEL /fit.itb#cfg2

Configuration can be use also for overlay management :
 KERNEL /fit.itb#cfg1#dtbo1#dtbo3

see doc/uImage.FIT/overlay-fdt-boot.txt for details

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>