linux: fix recursively defined variable

When running 'make printvars', the output stops at the time we dump the
Linux related variables, with:

    linux/linux.mk:109: *** Recursive variable `LINUX_TARGET_NAME'
    references itself (eventually).  Stop.

And that's expected, since we have:

    109 LINUX_TARGET_NAME = $(LINUX_IMAGE_NAME)
    [...]
    112 ifeq ($(LINUX_IMAGE_NAME),)
    113 LINUX_IMAGE_NAME = $(LINUX_TARGET_NAME)
    114 endif

Even though they are defined in a way that ensures they are in fact not
recursively defined (the if-block ensures that), 'printvars' does dump
all our variables by evaluating all of them, which in that specific case
implies they are recursively defined.

Fix that by explicitly setting LINUX_IMAGE_NAME in each if-block.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>