riscv: Use the generic src/lib/bootblock.c

TEST=Compiled for and ran on spike; it booted as before.

Change-Id: Id173643a3571962406f9191db248b206235dca35
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/16995
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/arch/riscv/Kconfig

@@ -6,7 +6,6 @@ config ARCH_BOOTBLOCK_RISCV
 	bool
 	default n
 	select ARCH_RISCV
-	select BOOTBLOCK_CUSTOM
 	select C_ENVIRONMENT_BOOTBLOCK
 
 config ARCH_VERSTAGE_RISCV