Makefile: Add a check of CONFIG_TRIM_UNUSED_KSYMS before remaking vmlinux

Gaurd the rule to re generate vmlinux only when CONFIG_TRIM_UNUSED_KSYMS
is set. Without this change, the vmlinux was generated twice as opposed to
Image generating only once.

Change-Id: I8370f64553b0ed7cb6536aabb1c68d77e5b58bf1
Signed-off-by: Prakruthi Deepak Heragu <pheragu@codeaurora.org>