uboot-phytec

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>