powerpc: P2020RDB-PC: Separate from P1_P2_RDB_PC in Kconfig

Use TARGET_P2020RDB_PC instead of sharing with P1_P2_RDB_PC to
simplify Kconfig and config macros.

Remove macro CONFIG_P2020RDB.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/board/freescale/p1_p2_rdb_pc/Kconfig b/board/freescale/p1_p2_rdb_pc/Kconfig
index f02549e..fb506d4 100644
--- a/board/freescale/p1_p2_rdb_pc/Kconfig
+++ b/board/freescale/p1_p2_rdb_pc/Kconfig
@@ -5,7 +5,8 @@
 	TARGET_P1020UTM		|| \
 	TARGET_P1021RDB		|| \
 	TARGET_P1024RDB		|| \
-	TARGET_P1025RDB
+	TARGET_P1025RDB		|| \
+	TARGET_P2020RDB
 
 config SYS_BOARD
 	default "p1_p2_rdb_pc"