cmd: add spawn and wait commands

Add a spawn command which runs another command in the background, as
well as a wait command to suspend the shell until one or more background
jobs have completed. The job_id environment variable is set by spawn and
wait accepts optional job ids, so that one can selectively wait on any
job.

Example:

 => date; spawn sleep 5; spawn sleep 3; date; echo "waiting..."; wait; date
 Date: 2025-02-21 (Friday)    Time: 17:04:52
 Date: 2025-02-21 (Friday)    Time: 17:04:52
 waiting...
 Date: 2025-02-21 (Friday)    Time: 17:04:57
 =>

Another example showing how background jobs can make initlizations
faster. The board is i.MX93 EVK, with one spinning HDD connected to
USB1 via a hub, and a network cable plugged into ENET1.

 # From power up / reset
 u-boot=> setenv autoload 0
 u-boot=> setenv ud "usb start; dhcp"
 u-boot=> time run ud
 [...]
 time: 8.058 seconds

 # From power up / reset
 u-boot=> setenv autoload 0
 u-boot=> setenv ud "spawn usb start; spawn dhcp; wait"
 u-boot=> time run ud
 [...]
 time: 4.475 seconds

Signed-off-by: Jerome Forissier <[email protected]>
Acked-by: Ilias Apalodimas <[email protected]>

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