u-boot: send hardening warning messages to stderr

[rborn-tx]

Send most of the hardening warning/error messages to U-Boot's stderr to increase the chances they are seen.

Related-to: #64

[jsrc27]

Do you think the following warning/error messages also need to be sent to stderr?:

meta-toradex-security/recipes-bsp/u-boot/files/0001-toradex-common-add-command-whitelisting-modules.patch

Line 1093 in 2c5ff36

+ printf("whitelist feature requires a valid FDT blob\n");

meta-toradex-security/recipes-bsp/u-boot/files/0004-toradex-integrate-bootm-protection-downstream.patch

Line 128 in 2c5ff36

+ puts("ERROR: bootm only accepts booting from a "

(same on upstream)

meta-toradex-security/recipes-bsp/u-boot/files/0004-toradex-integrate-bootm-protection-downstream.patch

Line 103 in 2c5ff36

+ puts("ERROR: can't boot from non-FIT images with "

(same on upstream)

Or are these different compared to the messages changed in this PR?

[rborn-tx]

Do you think the following warning/error messages also need to be sent to stderr?:

@jsrc27 Good question!

Basically in this PR I chose to send to stderr only messages starting with a double hash mark; those are the messages I considered (during development) as being important and more likely to happen (of course based on personal guess).

The messages you highlighted of course are important (same as hundreds of other errors U-Boot might detect, unrelated to the hardening) but they should be detected during development in which case I'd expect nobody to be executing U-Boot with stdout redirected to nulldev for example.

[jsrc27]

@rborn-tx Okay that makes sense just wanted to check/confirm. No other comments from my side then.

[sergioprado]

LGTM