menuconfig: print more info for symbol without prompts

When we search a config symbol, if it has no prompt the position of this
symbol in the Kconfig file and it's dependencies are not printed.  This
can be inconvenient, especially when it's set to n and we want to find out
why.

the following is an example:

before:

Symbol: GENERIC_SMP_IDLE_THREAD [=y]
Type  : boolean
  Selected by: X86 [=y]

after:

Symbol: GENERIC_SMP_IDLE_THREAD [=y]
Type  : boolean
  Defined at arch/Kconfig:213
  Selected by: X86 [=y]

Signed-off-by: Weng Meiling <wengmeiling.weng@huawei.com>
Signed-off-by: Libo Chen <libo.chen@huawei.com>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>