CODINGSTYLE: Drop non-posix parameter extensions

While the non-posix substring expansions and replacement expansions are
in use in portage already; they currently do not always work properly.

.githooks/pre-commit: 28: testing/gcc-cross-embedded/APKBUILD: Bad substitution

where line 28 reads:
	targetnorm="${target//-/_}"

So best to avoid this construct when it doesn't even work all the time.

A good replacement is:
	targetnorm="$(echo "${target}" | tr - _)"

which is posix compliant and works.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
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