kboot: Add powerpc stub for enumerate_memory_arch()

Add stub for new MI interface for enumerating memory. Right now powerpc
looks in the FDT table at a later point in boot since we don't need to
pass a specific memory table to the kernel. Leave it like that for now,
but note plans for the future.

Sponsored by:		Netflix
Reviewed by:		kevans
Differential Revision:	https://reviews.freebsd.org/D38248

stand/kboot/arch/powerpc64/load_addr.c

@@ -208,3 +208,15 @@ kboot_get_kernel_machine_bits(void)
 	return (bits);
 }
 #endif
+
+/* Need to transition from current hacky FDT way to this code */
+bool enumerate_memory_arch(void)
+{
+	/*
+	 * For now, we dig it out of the FDT, plus we need to pass all data into
+	 * the kernel via the (adjusted) FDT we find.
+	 */
+	setenv("usefdt", "1", 1);
+
+	return true;
+}

stand/kboot/main.c

@@ -254,7 +254,6 @@ main(int argc, const char **argv)
 	printf("\n%s", bootprog_info);
 
 	setenv("LINES", "24", 1);
-	setenv("usefdt", "1", 1);
 
 	memory_limits();