The current boot code prints too much error messages without much value
in them. This patch prints the error string when it makes sense and
simplifies the return path of bootscript_boot(). Make use of pr_err()
in boot_entry() which is the shared code responsible for calling the
boot_entry->boot callback. Remove the unnecessary print statement
from bootsource_action() as well.

This removes a single print statement ATM, allowing us to go from this:

    booting 'sd'
    Cannot set parameter mci0.probe: No such device
    running automount command 'mci${global.sd}.probe=1 && mount /dev/disk${global.sd}.0 /mnt/sd' failed
    could not open /mnt/sd/zImage: No such device
    ERROR: Booting 'sd' failed: No such device
    booting 'sd' failed: No such device
    boot: No such device

to this:

    Booting entry 'sd'
    Cannot set parameter mci0.probe: No such device
    running automount command 'mci${global.sd}.probe=1 && mount /dev/disk${global.sd}.0 /mnt/sd' failed
    could not open /mnt/sd/zImage: No such device
    ERROR: Booting entry 'sd' failed
    boot: No such device

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>