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/configs/P2020RDB-PC_NAND_defconfig b/configs/P2020RDB-PC_NAND_defconfig
index 4af6039..fd36351c 100644
--- a/configs/P2020RDB-PC_NAND_defconfig
+++ b/configs/P2020RDB-PC_NAND_defconfig
@@ -2,12 +2,12 @@
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_MPC85xx=y
-CONFIG_TARGET_P1_P2_RDB_PC=y
+CONFIG_TARGET_P2020RDB=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="P2020RDB,NAND"
+CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_BOOTDELAY=10
 CONFIG_SPL=y
 CONFIG_TPL=y