common: Only build cli_readline.o for CMDLINE on non-SPL

With gcc-6 and later we may get a warning such as:
.../common/cli_readline.c:20:21: warning: ‘tab_seq’ defined but not used [-Wunused-const-variable=]
 static const char   tab_seq[] = "        "; /* used to expand TABs */
                     ^~~~~~~
.../common/cli_readline.c:19:19: warning: ‘erase_seq’ defined but not used [-Wunused-const-variable=]
 static const char erase_seq[] = "\b \b"; /* erase sequence */
                   ^~~~~~~~~

Because in SPL we're normally not doing interactive commands anyhow, so
lets just not compile this at all in SPL.  This also means that we need
to correct the logic (and comment) about what the drivers/ddr/fsl/ and
CONFIG_FSL_DDR_INTERACTIVE requires and this will be included in SPL
there.

Signed-off-by: Tom Rini <trini@konsulko.com>