kboot: Implement mount(2)

Create a wrapper for the mount system call. To ensure a sane early boot
environment and to gather data we need for kexec, we may need to mount
some special filesystems.

Sponsored by:		Netflix

stand/kboot/arch/amd64/syscall_nr.h

@@ -7,6 +7,7 @@
 #define SYS_lseek		  8
 #define	SYS_mkdirat		258
 #define SYS_mmap		  9
+#define SYS_mount		165
 #define	SYS_munmap		 11
 #define SYS_newfstat		  5
 #define SYS_newfstatat		262

stand/kboot/arch/powerpc64/syscall_nr.h

@@ -8,6 +8,7 @@
 #define SYS_llseek		140
 #define	SYS_mkdirat		287
 #define SYS_mmap		 90
+#define SYS_mount		 21
 #define	SYS_munmap		 91
 #define SYS_newfstat		SYS_fstat
 #define SYS_newfstatat		291

stand/kboot/host_syscall.h

@@ -88,6 +88,9 @@ struct host_timeval {
 
 #define HOST_AT_FDCWD		-100		/* Relative to current directory */
 
+/* Mount flags from uapi */
+#define MS_RELATIME (1 << 21)
+
 /*
  * System Calls
  */
@@ -101,6 +104,8 @@ int host_kexec_load(uint32_t start, int nsegs, uint32_t segs, uint32_t flags);
 ssize_t host_llseek(int fd, int32_t offset_high, int32_t offset_lo, uint64_t *result, int whence);
 int host_mkdir(const char *, host_mode_t);
 void *host_mmap(void *addr, size_t len, int prot, int flags, int fd, off_t off);
+int host_mount(const char *src, const char *target, const char *type,
+    unsigned long flags, void *data);
 int host_munmap(void *addr, size_t len);
 int host_open(const char *path, int flags, int mode);
 ssize_t host_read(int fd, void *buf, size_t nbyte);

stand/kboot/host_syscalls.c

@@ -75,6 +75,13 @@ host_mmap(void *addr, size_t len, int prot, int flags, int fd, off_t off)
 	return (void *)host_syscall(SYS_mmap, (uintptr_t)addr, len, prot, flags, fd, off);
 }
 
+int
+host_mount(const char *src, const char *target, const char *type, unsigned long flags,
+    void *data)
+{
+	return host_syscall(SYS_mount, src, target, type, flags, data);
+}
+
 int
 host_munmap(void *addr, size_t len)
 {