kconfig: report recursive dependency involving 'imply'

Currently, Kconfig does not complain about the recursive dependency
where 'imply' keywords are involved.

[Test Code]

config A
bool "a"

config B
bool "b"
imply A
depends on A

In the code above, Kconfig cannot calculate the symbol values correctly
due to the circular dependency. For example, allyesconfig followed by
syncconfig results in an odd behavior because CONFIG_B becomes visible
in syncconfig.

$ make allyesconfig
scripts/kconfig/conf --allyesconfig Kconfig
#
# configuration written to .config
#
$ cat .config
#
# Automatically generated file; DO NOT EDIT.
# Main menu
#
CONFIG_A=y
$ make syncconfig
scripts/kconfig/conf --syncconfig Kconfig
*
* Restart config...
*
*
* Main menu
*
a (A) [Y/n/?] y
b (B) [N/y/?] (NEW)

To detect this correctly, sym_check_expr_deps() should recurse to
not only sym->rev_dep.expr but also sym->implied.expr .

At this moment, sym_check_print_recursive() cannot distinguish
'select' and 'imply' since it does not know the precise context
where the recursive dependency has been hit. This will be solved
by the next commit.

In fact, even the document and the unit-test are confused. Using
'imply' does not solve recursive dependency since 'imply' addresses
the unmet direct dependency, which 'select' could cause.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Dirk Gouders <dirk@gouders.net>

kconfig: error out when seeing recursive dependency

Originally, recursive dependency was a fatal error for Kconfig
because Kconfig cannot compute symbol values in such a situation.

Commit d595cea62403 ("kconfig: print more info when we see a recursive
dependency") changed it to a warning, which I guess was not intentional.

Get it back to an error again.

Also, rename the unit test directory "warn_recursive_dep" to
"err_recursive_dep" so that it matches to the behavior.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Dirk Gouders <dirk@gouders.net>