default to iconv from C library for h8/300

target/config/Config.in.toolchain

@@ -4,6 +4,7 @@
 choice
 prompt "Iconv implementation"
 bool
+default ADK_TARGET_LIBC_ICONV if ADK_TARGET_ARCH_H8300
 default ADK_TARGET_LIBICONV_TINY if ADK_TARGET_LIB_UCLIBC_NG
 
 config ADK_TARGET_LIBC_ICONV