dfu: Remove dependency on HUSH parser in SPL

dfu: Remove dependency on HUSH parser in SPL

CLI support with the HUSH parser is not currently SPL safe due to it's
use of realloc. That function is not defined for SPLs that use
SYS_MALLOC_SIMPLE. CLI support can be built in to SPL and some functions
do work, but use of some like run_command() will cause build to fail.
When no SPL code calls this function build works as the compiler removes
this unreachable code so the unresolved symbols are ignored.

If DFU support is enabled in SPL then MMU DFU support may get brought in
also, this code does make a call to run_command() causing build to fail
if the HUSH parser is not built-in. To break this odd and unneeded
dependency chain we use CONFIG_IS_ENABLED where appropriate to prevent
calls into HUSH code from SPL. This also removes our need to pull in the
rather unrelated source file when SPL_DFU is defined.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>