PowerPC: Update MIP405/MIP405T to use Kconfig better

Convert CONFIG_MIP405T from SYS_EXTRA_OPTIONS to a real config

There are two boards, MIP405 and MIP405T that have a few differences.
Start by checking for CONFIG_TARGET_MIP405.  Then introduce
CONFIG_TARGET_MIP405T and use that not CONFIG_MIP405T.  Next, convert
also convert the usage of CONFIG_ISO_STRING to be based on Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/powerpc/cpu/ppc4xx/resetvec.S b/arch/powerpc/cpu/ppc4xx/resetvec.S
index b3308bd..a42d91f 100644
--- a/arch/powerpc/cpu/ppc4xx/resetvec.S
+++ b/arch/powerpc/cpu/ppc4xx/resetvec.S
@@ -4,7 +4,8 @@
 #if defined(CONFIG_440)
 	b _start_440
 #else
-#if defined(CONFIG_BOOT_PCI) && defined(CONFIG_MIP405)
+#if defined(CONFIG_BOOT_PCI) && (defined(CONFIG_TARGET_MIP405) \
+				 || defined(CONFIG_TARGET_MIP405T))
 	b _start_pci
 #else
 	b _start