boot: zephyr: Fix Kconfig inclusion

According to our CI:

Redundant 'source "$(ZEPHYR_BASE)/Kconfig.zephyr" in
'boot/zephyr/Kconfig'. Just do 'source "Kconfig.zephyr"'
instead. The $srctree environment variable already points
to the Zephyr root, and all 'source's are relative to it.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>