powerpc: P1021RDB: Separate from P1_P2_RDB_PC in Kconfig

Use TARGET_P1021RDB instead of sharing with TARGET_P1_P2_RDB_PC to
simplify Kconfig and macros.

Remove macro CONFIG_P1021RDB.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/configs/P1021RDB-PC_NAND_defconfig b/configs/P1021RDB-PC_NAND_defconfig
index ebfc053..29335b1 100644
--- a/configs/P1021RDB-PC_NAND_defconfig
+++ b/configs/P1021RDB-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_P1021RDB=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="P1021RDB,NAND"
+CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_BOOTDELAY=10
 CONFIG_SPL=y
 CONFIG_TPL=y