Move CONFIG_PANIC_HANG to Kconfig

Freescale (NXP) boards have lots of defconfig files per board.
I used "imply PANIC_HANG" for them.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: York Sun <york.sun@nxp.com>
diff --git a/configs/xpedite517x_defconfig b/configs/xpedite517x_defconfig
index a2324c3..3773518 100644
--- a/configs/xpedite517x_defconfig
+++ b/configs/xpedite517x_defconfig
@@ -27,4 +27,5 @@
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_PHYLIB=y
 CONFIG_SYS_NS16550=y
+CONFIG_PANIC_HANG=y
 CONFIG_OF_LIBFDT=y