kconfig: do not overwrite symbol direct dependency in assignment

Considering the following configuration:

config F
    bool "F"

choice AB
    bool "AB"
config A
    bool "A"
config B
    bool "B"
endchoice

if A
config D
    bool
    default y if F
    select E
config E
    bool "E"
endif

if B
config D
    bool
    default y if F
    select E
config E
    bool "E"
endif

The following configuration:

 CONFIG_F=y
 CONFIG_A=y
 # CONFIG_B is not set
 CONFIG_D=y
 CONFIG_E=y

emits a spurious warning:

(D) selects E which has unmet direct dependencies (B)

If a symbol appears in two different branch of the tree, it should inherit the
dependency of both parent, not just the last one.

Reported-by: Yann E. Morin <yann.morin.1998@anciens.enib.fr>
Tested-by: Yann E. Morin <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>