cmake: fixed issue with CONF_FILE pattern when using build type

This commit fixes an issue where specifying a build type config file
in a subfolder named: `prj_<something>` would cause a wrong match and
fail to find Kconfig fragments, as example:

Working: `-DCONF_FILE=myconfigs/prj_debug.conf` would correctly add
myconfigs/boards/<board>_debug.conf to list of conf files.

Failing: `-DCONF_FILE=prj_configs/prj_debug.conf` would fail to locate
prj_configs/boards/<board>_debug.conf, and thus the board specific
config fragment was not added to the list of config files.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>