makesyscalls: don't make syscall.mk by default

We only want to produce syscall.mk for the main syscall table so default
to not producing it (send it to /dev/null) and add a syscalls.conf to
sys/kern to trigger the creation of sys/sys/syscall.mk.  This eliminates
the need for entries in other syscalls.conf files and is a cleaner
pattern going forward.

Reviewed by:	kevans, imp
Differential Revision:	https://reviews.freebsd.org/D42663

sys/amd64/linux/syscalls.conf

@@ -3,7 +3,6 @@ sysproto="linux_proto.h"
 sysproto_h=_LINUX_SYSPROTO_H_
 syshdr="linux_syscall.h"
 syssw="linux_sysent.c"
-sysmk="/dev/null"
 syscallprefix="LINUX_SYS_"
 switchname="linux_sysent"
 namesname="linux_syscallnames"

sys/amd64/linux32/syscalls.conf

@@ -3,7 +3,6 @@ sysproto="linux32_proto.h"
 sysproto_h=_LINUX32_SYSPROTO_H_
 syshdr="linux32_syscall.h"
 syssw="linux32_sysent.c"
-sysmk="/dev/null"
 syscallprefix="LINUX32_SYS_"
 switchname="linux32_sysent"
 namesname="linux32_syscallnames"

sys/arm64/linux/syscalls.conf

@@ -3,7 +3,6 @@ sysproto="linux_proto.h"
 sysproto_h=_LINUX_SYSPROTO_H_
 syshdr="linux_syscall.h"
 syssw="linux_sysent.c"
-sysmk="/dev/null"
 syscallprefix="LINUX_SYS_"
 switchname="linux_sysent"
 namesname="linux_syscallnames"

sys/compat/freebsd32/syscalls.conf

@@ -3,7 +3,6 @@ sysproto="freebsd32_proto.h"
 sysproto_h=_FREEBSD32_SYSPROTO_H_
 syshdr="freebsd32_syscall.h"
 syssw="freebsd32_sysent.c"
-sysmk="/dev/null"
 syscallprefix="FREEBSD32_SYS_"
 switchname="freebsd32_sysent"
 namesname="freebsd32_syscallnames"

sys/i386/linux/syscalls.conf

@@ -3,7 +3,6 @@ sysproto="linux_proto.h"
 sysproto_h=_LINUX_SYSPROTO_H_
 syshdr="linux_syscall.h"
 syssw="linux_sysent.c"
-sysmk="/dev/null"
 syscallprefix="LINUX_SYS_"
 switchname="linux_sysent"
 namesname="linux_syscallnames"

sys/kern/Makefile

@@ -2,7 +2,6 @@
 #
 # Makefile for init_sysent
 
-SYSENT_CONF=
 GENERATED=	init_sysent.c			\
 		syscalls.c			\
 		systrace_args.c			\

sys/kern/syscalls.conf

@@ -0,0 +1 @@
+sysmk="../sys/syscall.mk"

sys/tools/makesyscalls.lua

@@ -46,7 +46,7 @@ local config = {
 	sysproto = "../sys/sysproto.h",
 	sysproto_h = "_SYS_SYSPROTO_H_",
 	syshdr = "../sys/syscall.h",
-	sysmk = "../sys/syscall.mk",
+	sysmk = "/dev/null",
 	syssw = "init_sysent.c",
 	syscallprefix = "SYS_",
 	switchname = "sysent",