xlat/gen.sh: introduce #Prefix directive

This directive is intended for other tools and is currently ignored
by `gen.sh`, similar to #From and #Generated.

* src/xlat/gen.sh: Introduce #Prefix.
* src/xlat/README.md: Document it.

Author: ldv-alt

src/xlat/README.md

@@ -183,6 +183,22 @@ Annotates the xlat file with the source of the xlat constants.  There could be
 several `#From` directives in a single xlat file.  This directive is intended
 for other tools and is currently ignored by `gen.sh`.
 
+### `#Prefix PREFIX1_ ... PREFIXn_`
+
+Annotates the xlat file with the prefix pattern(s) to use when searching for
+constants in Linux headers.  Multiple prefixes can be specified, separated by
+spaces.  This directive is intended for other tools and is currently ignored
+by `gen.sh`.
+
+**Example:**
+```
+#From include/uapi/linux/io_uring.h
+#Prefix IORING_REGISTER_ IORING_UNREGISTER_
+IORING_REGISTER_BUFFERS
+IORING_UNREGISTER_BUFFERS
+...
+```
+
 ### `#<anything else>`
 
 Any other line starting with `#` is passed through verbatim.  Allows custom

src/xlat/gen.sh

@@ -218,7 +218,7 @@ gen_header()
 		esac
 
 		case $line in
-		'#From '*|'#Generated '*) # ignore
+		'#From '*|'#Generated '*|'#Prefix '*) # ignore
 			;;
 		'#conditional')
 			unconditional=
@@ -322,7 +322,7 @@ gen_header()
 		esac
 
 		case ${line} in
-		'#From '*|'#Generated '*) # ignore
+		'#From '*|'#Generated '*|'#Prefix '*) # ignore
 			;;
 		'#conditional')
 			unconditional=