bootstd: Rename distro and syslinux to extlinux

We use the terms 'distro' to mean extlinux but they are not really the
same. 'Distro' could refer to any method of booting a distribution,
whereas extlinux is a particular method.

Also we sometimes use syslinux, but it is better to use the same term in
all cases.

Rename distro to syslinux and also update bootstd uses of syslinux to use
extlinux instead.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/develop/bootstd.rst

@@ -154,7 +154,7 @@ bootmeths
 This environment variable can be used to control the list of bootmeths used and
 their ordering for example::
 
-   setenv bootmeths "syslinux efi"
+   setenv bootmeths "extlinux efi"
 
 Entries may be removed or re-ordered in this list to affect the order the
 bootmeths are tried on each bootdev. If the variable is empty, the default
@@ -389,8 +389,8 @@ Configuration
 -------------
 
 Standard boot is enabled with `CONFIG_BOOTSTD`. Each bootmeth has its own CONFIG
-option also. For example, `CONFIG_BOOTMETH_DISTRO` enables support for distro
-boot from a disk.
+option also. For example, `CONFIG_BOOTMETH_EXTLINUX` enables support for
+booting from a disk using an `extlinux.conf` file.
 
 To enable all feature sof standard boot, use `CONFIG_BOOTSTD_FULL`. This
 includes the full set of commands, more error messages when things go wrong and
@@ -406,8 +406,8 @@ Available bootmeth drivers
 
 Bootmeth drivers are provided for:
 
-   - distro boot from a disk (syslinux)
-   - distro boot from a network (PXE)
+   - extlinux / syslinux boot from a disk
+   - extlinux boot from a network (PXE)
    - U-Boot scripts from disk, network or SPI flash
    - EFI boot using bootefi from disk
    - VBE
@@ -683,8 +683,8 @@ This feature can be added as needed. Note that sandbox is a special case, since
 in that case the host filesystem can be accessed even though the block device
 is NULL.
 
-If we take the example of the `bootmeth_distro` driver, this call ends up at
-`distro_read_bootflow()`. It has the filesystem ready, so tries various
+If we take the example of the `bootmeth_extlinux` driver, this call ends up at
+`extlinux_read_bootflow()`. It has the filesystem ready, so tries various
 filenames to try to find the `extlinux.conf` file, reading it if possible. If
 all goes well the bootflow ends up in the `BOOTFLOWST_READY` state.
 
@@ -697,12 +697,12 @@ the  `BOOTFLOWST_READY` state.
 
 That is the basic operation of scanning for bootflows. The process of booting a
 bootflow is handled by the bootmeth driver for that bootflow. In the case of
-distro boot, this parses and processes the `extlinux.conf` file that was read.
-See `distro_boot()` for how that works. The processing may involve reading
+extlinux boot, this parses and processes the `extlinux.conf` file that was read.
+See `extlinux_boot()` for how that works. The processing may involve reading
 additional files, which is handled by the `read_file()` method, which is
-`distro_read_file()` in this case. All bootmethds should support reading files,
-since the bootflow is typically only the basic instructions and does not include
-the operating system itself, ramdisk, device tree, etc.
+`extlinux_read_file()` in this case. All bootmethds should support reading
+files, since the bootflow is typically only the basic instructions and does not
+include the operating system itself, ramdisk, device tree, etc.
 
 The vast majority of the bootstd code is concerned with iterating through
 partitions on bootdevs and using bootmethds to find bootflows.