stand: userboot: allow building on !x86

We can still get plenty of use out of a userboot that doesn't know
anything about how to load or boot a kernel; notably, the test harness
in tools/boot can still be used to test lua changes.

Hack out the necessary bits to simply build on other platforms, and add
a small warning with ample time to view the warning on other platforms.
We still won't build userboot by default on these platforms, since the
build product isn't useful for most people.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D41529

stand/userboot/userboot/Makefile

@@ -13,18 +13,30 @@ SHLIB_NAME=	userboot_${LOADER_INTERP}.so
 STRIP=
 LIBDIR=		/boot
 
+.if ${MACHINE_CPUARCH} == "amd64"
+USERBOOT_KERNEL_SUPPORT=	yes
+.else
+USERBOOT_KERNEL_SUPPORT=	no
+.endif
+
 .PATH:		${.CURDIR}/../userboot
 SRCS=		autoload.c
 SRCS+=		bcache.c
+.if ${MACHINE_CPUARCH} == "amd64"
 SRCS+=		biossmap.c
+.endif
+.if ${USERBOOT_KERNEL_SUPPORT} == "yes"
 SRCS+=		bootinfo.c
 SRCS+=		bootinfo32.c
 SRCS+=		bootinfo64.c
+.endif
 SRCS+=		conf.c
 SRCS+=		copy.c
 SRCS+=		devicename.c
+.if ${USERBOOT_KERNEL_SUPPORT} == "yes"
 SRCS+=		elf32_freebsd.c
 SRCS+=		elf64_freebsd.c
+.endif
 SRCS+=		host.c
 SRCS+=		main.c
 SRCS+=		userboot_cons.c
@@ -49,6 +61,10 @@ VERSION_FILE?=	${.CURDIR}/../userboot/version
 LINKS+=		${BINDIR}/${SHLIB_NAME} ${BINDIR}/userboot.so
 .endif
 
+.if ${USERBOOT_KERNEL_SUPPORT} == "yes"
+CFLAGS+=	-DUSERBOOT_KERNEL_SUPPORT
+.endif
+
 .if ${MK_LOADER_ZFS} != "no"
 CFLAGS+=	-DUSERBOOT_ZFS_SUPPORT
 HAVE_ZFS=yes

stand/userboot/userboot/conf.c

@@ -83,16 +83,20 @@ struct netif_driver *netif_drivers[] = {
  * Sort formats so that those that can detect based on arguments
  * rather than reading the file go first.
  */
+#if defined(__amd64__)
 extern struct file_format	i386_elf;
 extern struct file_format	i386_elf_obj;
 extern struct file_format	amd64_elf;
 extern struct file_format	amd64_elf_obj;
+#endif
 
 struct file_format *file_formats[] = {
+#if defined(__amd64__)
     &i386_elf,
     &i386_elf_obj,
     &amd64_elf,
     &amd64_elf_obj,
+#endif
     NULL
 };
 

stand/userboot/userboot/main.c

@@ -205,6 +205,11 @@ loader_main(struct loader_callbacks *cb, void *arg, int version, int ndisks)
 	devinit();
 	extract_currdev();
 
+#if !defined(USERBOOT_KERNEL_SUPPORT)
+	printf("WARNING: This userboot does not support loading a kernel\n");
+	delay(1500000);
+#endif
+
 	/*
 	 * Checking the interpreter isn't worth the overhead unless we
 	 * actually have the swap_interpreter callback, so we actually version