powerpc: P1020MBG: Separate from P1_P2_RDB_PC in Kconfig

Use TARGET_P1020MBG instead of sharing with TARGET_P1_P2_RDB_PC to
simplify Kconfig and other macros.

Remove macro CONFIG_P1020MBG.

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 d3352d2..2c45560 100644
--- a/board/freescale/p1_p2_rdb_pc/Kconfig
+++ b/board/freescale/p1_p2_rdb_pc/Kconfig
@@ -1,4 +1,5 @@
-if TARGET_P1_P2_RDB_PC
+if TARGET_P1_P2_RDB_PC || \
+	TARGET_P1020MBG
 
 config SYS_BOARD
 	default "p1_p2_rdb_pc"