arm, powerpc: select SYS_GENERIC_BOARD

We have finished Generic Board conversion for ARM and PowerPC, i.e.
all the boards have been converted except OpenRISC, SuperH, SPARC,
which have not supported Generic Board framework yet.

Select SYS_GENERIC_BOARD in arch/Kconfig and delete all the macro
defines in include/configs/*.h.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>