Define prototype for exit and ensure references

Define a prototype for exit in stand.h. Provide a reference to exit in
a few conf.c files to ensure that its definition gets pulled in early.
Since exit() is a MD routine, it isn't defined in libsa. However,
libsa tends to be listed last and will soon have panic() in it which
calls exit(). The reference to exit early ensures that the MD exit is
available to satisfy linking for static libraries.

sys/boot/arm/uboot/conf.c

@@ -36,6 +36,9 @@ __FBSDID("$FreeBSD$");
 #include "dev_net.h"
 #endif
 
+/* Make sure we have an explicit reference to exit so libsa's panic pulls in the MD exit */
+void (*exitfn)(int) = exit;
+
 struct devsw *devsw[] = {
 #if defined(LOADER_DISK_SUPPORT) || defined(LOADER_CD9660_SUPPORT)
 	&uboot_storage,

sys/boot/libsa/stand.h

@@ -376,6 +376,7 @@ extern int	null_readdir(struct open_file *f, struct dirent *d);
  * Machine dependent functions and data, must be provided or stubbed by 
  * the consumer 
  */
+extern void		exit(int);
 extern int		getchar(void);
 extern int		ischar(void);
 extern void		putchar(int);

sys/boot/mips/uboot/conf.c

@@ -36,6 +36,9 @@ __FBSDID("$FreeBSD$");
 #include "dev_net.h"
 #endif
 
+/* Make sure we have an explicit reference to exit so libsa's panic pulls in the MD exit */
+void (*exitfn)(int) = exit;
+
 struct devsw *devsw[] = {
 #if defined(LOADER_DISK_SUPPORT) || defined(LOADER_CD9660_SUPPORT)
 	&uboot_storage,

sys/boot/powerpc/ofw/conf.c

@@ -36,6 +36,9 @@ __FBSDID("$FreeBSD$");
 #include "dev_net.h"
 #endif
 
+/* Make sure we have an explicit reference to exit so libsa's panic pulls in the MD exit */
+void (*exitfn)(int) = exit;
+
 /*
  * We could use linker sets for some or all of these, but
  * then we would have to control what ended up linked into

sys/boot/powerpc/uboot/conf.c

@@ -35,6 +35,9 @@ __FBSDID("$FreeBSD$");
 #include "dev_net.h"
 #endif
 
+/* Make sure we have an explicit reference to exit so libsa's panic pulls in the MD exit */
+void (*exitfn)(int) = exit;
+
 /*
  * We could use linker sets for some or all of these, but
  * then we would have to control what ended up linked into