bootm: Change incorrect 'unsupported' error

bootm: Change incorrect 'unsupported' error

At present when bootm fails, it says:

    subcommand not supported

and then prints help for the bootm command. This is not very useful, since
generally the error is related to something else, such as fixups failing.
It is quite confusing to see this in a test run.

Change the error and show the error code.

We could update the OS functions to return -ENOSYS when they do not
support the bootm subcommand. But this involves some thought since this is
arch-specific code and proper errno error codes are not always returned.
Also, with the code as is, all required subcommands are of course
supported - a problem would only come if someone added a new one or
removed support for one from an existing OS. Therefore it seems better to
leave that sort of effort for when our bootm tests are improved.

Note: v1 of this patch generated a discussion[1] about printing error
strings automatically using printf(). That is outside the scope of this
patch but will be dealt with separately.

[1] https://patchwork.ozlabs.org/project/uboot/patch/20220909151801.336551-3-sjg@chromium.org/

Signed-off-by: Simon Glass <sjg@chromium.org>