powerpc: P1020RDB-PC: Separate from P1_P2_RDB_PC in Kconfig

Use TARGET_P1020RDB_PC instead of sharing with TARGET_P1_P2_RDB_PC
to simplify Kconfig and config macros.

Remove macro CONFIG_P1020RDB_PC.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/include/configs/p1_p2_rdb_pc.h b/include/configs/p1_p2_rdb_pc.h
index 25f1967..edb5395 100644
--- a/include/configs/p1_p2_rdb_pc.h
+++ b/include/configs/p1_p2_rdb_pc.h
@@ -30,7 +30,7 @@
 #define CONFIG_SYS_L2_SIZE	(256 << 10)
 #endif
 
-#if defined(CONFIG_P1020RDB_PC)
+#if defined(CONFIG_TARGET_P1020RDB_PC)
 #define CONFIG_BOARDNAME "P1020RDB-PC"
 #define CONFIG_NAND_FSL_ELBC
 #define CONFIG_P1020