fdt: boot_get_fdt: really boot w/o FDT when "goto no_fdt"

fdt: boot_get_fdt: really boot w/o FDT when "goto no_fdt"

The 'no_fdt' goto label was introduced by v2015.01 commit [0] and it
had two review stages [1-2]. The *documented* purpose behind commit [0]
is (excerpt from commit description):

> allows both FDT and non-FDT kernels to boot by making the
> third parameter to the bootm/bootz optional

While [1] and [2] share the same goal, they have very different
implementations:
 - [1] was based on a very simple 'argc' check at function error out
   with returning success to the caller if the third parameter was NOT
   passed to bootm/bootz command. This approach had the downside of
   returning success to the caller even in case of legitimate internal
   errors, which should halt booting.
 - [2] added the "no_fdt" label and several "goto no_fdt" statements.
   This allowed to report the legitimate internal errors to the caller.

IOW the major difference between [1] and [2] is:
 - [1] boot w/o FDT if FDT address is not passed to boot{m,z,*}
 - [2] give *freedom* to the developer to boot w/o FDT from any
   (more or less) arbitrary point in the function flow (and here
   comes the peculiar aspect, which looks to be a leftover from [1])
   with the precondition that the 3rd argument (FDT address) is NOT
   provided to boot{m,z,*}. In practice, this means that only a subset
   of "goto no_fdt" end up booting w/o FDT while the other subset is
   returning an error to the caller.

This patch removes the peculiar behavior described above, such that
"goto no_fdt" performs really what it tells to the developer.

The motivation of this patch is to decrease the unneeded complexity
and increase the readability of boot_get_fdt().

[0] 48aead71c1ad ("fdt: Allow non-FDT kernels to boot when CONFIG_OF_LIBFDT is defined")
[1] https://patchwork.ozlabs.org/patch/412923/
    ("[U-Boot,v1] fdt: Allow non-FDT kernels to boot when CONFIG_OF_LIBFDT is defined")
[2] https://patchwork.ozlabs.org/patch/415635/
    ("[U-Boot,v2] fdt: Allow non-FDT kernels to boot when CONFIG_OF_LIBFDT is defined")

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Simon Glass <sjg@chromium.org>