[main] Add the "scriptlet" setting

A scriptlet is a single iPXE command that can be stored in
non-volatile option storage and used to override the default
"autoboot" behaviour without having to reflash the iPXE image.

For example, a scriptlet could contain

    autoboot || reboot

to instruct iPXE to reboot the system if booting fails.

Unlike an embedded image, the presence of a scriptlet does not inhibit
the initial "Press Ctrl-B..." prompt.  This allows the user to recover
from setting a faulty scriptlet.

Originally-implemented-by: Glenn Brown <glenn@myri.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/main.c

 FILE_LICENCE ( GPL2_OR_LATER );
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <ipxe/init.h>
 #include <ipxe/features.h>
 #include <ipxe/shell.h>
 #define BOLD	"\033[1m"
 #define CYAN	"\033[36m"
 
+/** The "scriptlet" setting */
+struct setting scriptlet_setting __setting ( SETTING_MISC ) = {
+	.name = "scriptlet",
+	.description = "Boot scriptlet",
+	.tag = DHCP_EB_SCRIPTLET,
+	.type = &setting_type_string,
+};
+
 /**
  * Prompt for shell entry
  *
 __asmcall int main ( void ) {
 	struct feature *feature;
 	struct image *image;
+	char *scriptlet;
 
 	/* Some devices take an unreasonably long time to initialise */
 	printf ( PRODUCT_SHORT_NAME " initialising devices..." );
 	if ( ( image = first_image() ) != NULL ) {
 		/* We have an embedded image; execute it */
 		image_exec ( image );
+	} else if ( shell_banner() ) {
+		/* User wants shell; just give them a shell */
+		shell();
 	} else {
-		/* Prompt for shell */
-		if ( shell_banner() ) {
-			/* User wants shell; just give them a shell */
-			shell();
+		fetch_string_setting_copy ( NULL, &scriptlet_setting,
+					    &scriptlet );
+		if ( scriptlet ) {
+			/* User has defined a scriptlet; execute it */
+			system ( scriptlet );
+			free ( scriptlet );
 		} else {
 			/* Try booting.  If booting fails, offer the
 			 * user another chance to enter the shell.

src/include/ipxe/dhcp.h

 #define DHCP_EB_SKIP_SAN_BOOT DHCP_ENCAP_OPT ( DHCP_EB_ENCAP, 0x09 )
 
 /*
- * Tags in the range 0x10-0x7f are reserved for feature markers
+ * Tags in the range 0x10-0x4f are reserved for feature markers
  *
  */
 
+/** Scriptlet
+ *
+ * If a scriptlet exists, it will be executed in place of the usual
+ * call to autoboot()
+ */
+#define DHCP_EB_SCRIPTLET DHCP_ENCAP_OPT ( DHCP_EB_ENCAP, 0x51 )
+
 /** Skip PXE DHCP protocol extensions such as ProxyDHCP
  *
  * If set to a non-zero value, iPXE will not wait for ProxyDHCP offers