Clean up .gitignore now that we're using CMake

"Out of source" builds have been used since the build was switched to
CMake in 2019. So there's no need to ignore specific .o, .la and a
gazillion of other files extensions anymore. It's now enough to ignore
`build*` directories, no need to ignore files by extension any more.

Also remove personal ignore preferences like *.mbox, cscope* etc. They
belong to a centralized file in the HOME directory, not to a massive
copy/paste/diverge across all projects.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

.gitignore

@@ -1,50 +1,16 @@
-*.o
-*.Po
-*.so*
-*~
-*.a
-*.la
-*.lo
-*.Tpo
-*.swp
-*.bin
-*.map
-*.ri
-*.orig
-*.rej
-*.rom
-*.man
-.version
-*.x
-.build
-*.dis
-*.lst
-*.log
-*.html
-*.js
-*.png
-*.md5
-*.xml
-*.css
-*.xsd
-*.patch
-*.xslt
-*.mbox
+# Do not mass copy/paste/diverge in every project your non-project
+# specific, personal git ignore preferences like: *.mbox, *.patch, *.rej,
+# cscope*, TAGS, *~,...
+#
+# Instead, set your non-project specific, personal git ignore preference
+# only once for all git projects in your home directory:
+#     https://git-scm.com/docs/gitignore
+# So this does not interfere other people's "git status" preferences.
+#
+# Also, don't use `git add *` or `git commit -a` and git push blindly
+# without a look at which files are getting pushed. This is bad no
+# matter what is in git ignore.
 
-*.gz
-*.xz
-src/arch/xtensa/sof
-src/arch/xtensa/boot_ldr
-src/arch/xtensa/sof-*
-src/arch/xtensa/boot_ldr-*
-src/arch/xtensa/module
-src/arch/xtensa/rom
-src/arch/xtensa/rom-*
-boot_module
-
-tap-driver.sh
-test-driver
-*.trs
 tools/test/audio/reports/*
 tools/test/audio/zeros_in.raw
 
@@ -57,11 +23,3 @@ build*/
 .settings
 .project
 .cproject
-
-/tags
-/TAGS
-
-# cscope files
-cscope.*
-ncscope.*
-