fs/jffs2: Fix mkfs.jffs2 --pagesize parameter usage

Fixes #8186

Mkfs.jffs2 accepts a --pagesize parameter, which allows specifying the size
of the virtual memory page size of the target machine, where the image will
be used.  (This is the value of the PAGE_SIZE macro in Linux.) In most cases
the parameter doesn't need to be set as the default value of 4 kB is usually
correct.

The parameter was used incorrectly in Buildroot -- it was set to the page
size of flash memory chip -- this commit fixes this problem.  Now the
--pagesize parameter is not used at all (unless the user explicitly chooses
to use a custom value during configuration).  All existing defconfigs were
corrected to match the new configuration variable names.

[Peter: reword, add Config.in.legacy handling]
Signed-off-by: Michał Leśniewski <mlesniew@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>