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_36BIT_defconfig b/configs/P1021RDB-PC_36BIT_defconfig
index 6c1cb28..87d45b2 100644
--- a/configs/P1021RDB-PC_36BIT_defconfig
+++ b/configs/P1021RDB-PC_36BIT_defconfig
@@ -1,12 +1,11 @@
 CONFIG_PPC=y
 CONFIG_MPC85xx=y
-CONFIG_TARGET_P1_P2_RDB_PC=y
+CONFIG_TARGET_P1021RDB=y
 CONFIG_PHYS_64BIT=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="P1021RDB"
 CONFIG_BOOTDELAY=10
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_MMC=y