How to find out what to choose in the target system and target images?
Target system you can infer from the columns of the Target / Platform in the Table of Hardware :
http://wiki.openwrt.org/toh/start
Target images depend on the intended use of the assembly, but in most common cases (for example, to host on a router) you want either squashfs or jffs2 . See Link to file systems below.
Of all the created images, how do I know which one I should use (for example, WRT54GL is not listed above, how do I say whether to use "openwrt-wrt54g-jffs2.bin" correctly?
If your router is not listed among user images, you should probably use a generic image, for example: openwrt-brcm47xx-squashfs.trx
What criteria can I use to decide if I will use jffs or squashfs image?
From http://wiki.openwrt.org/doc/techref/filesystems :
SquashFS
+ takes up as little space as possible
+ , which allows you to implement the fix of the FailSafe idiot for recovery, since it cannot be written
- read only
- , since every time the file contained on it changes, in fact a copy of it is copied to the second section (JFFS2)
Jffs
+ recordable, has journaling and wear leveling
+ cool
- is compressed, so the program (in particular, opkg) cannot know in advance how much space the package takes,
Do not be fooled about the read-only part.
All OpenWrt firmware includes a completely rewritable root file system that will remain intact during a power outage. The confusion is that squashfs is a readonly file system; all OpenWrt firmware also includes a jffs2 section - the squashfs part of the file name applies only to the file system included in the firmware image; additional files or changes are saved on jffs2.
- The squashfs section will always contain all the files in the same way as when using the firmware; You cannot change them without reflashing. (see # 6)
- The jffs2 section contains only your changes to the file system; since squashfs still contains the original version, you can easily return the files back to their original state.
You can delete the squashfs section by installing the jffs2 firmware version, but this is not recommended - it uses more space and does not have the above fail-safe features.
Source: https://forum.openwrt.org/viewtopic.php?pid=36636#p36636
If in doubt, use squashfs per http://wiki.openwrt.org/doc/howto/obtain.firmware.download#filesystem