devtools: fix config check when building tags

devtools: fix config check when building tags

The check for a valid configuration in build-tags.sh relied on the output
of "make showconfig" rather than checking directly for a config file of
that name. This broke when as part of the rename of the linuxapp/bsdapp
configs to just linux/freebsd, as we stopped advertising the old names
even if they worked. Changing the code to just look for the config
file by name fixes this issue while shortening the code too.

Fixes: 218c4e68c1d9 ("mk: use linux and freebsd in config names")
Fixes: aafaea3d3b70 ("devtools: add tags and cscope index generation")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>