apk fetch --stdout will mix the progress bar with data output. We work
around this by run it in quiet mode.

Patch from Andrew Manison.

ref #626

This works around a bug in busybox 1.22.0 that makes 'grep -w ^' hang.
It is triggered when there are no deps. (eg. musl)

It means that home built packages cannot be installed with abuild-apk
unless the key is installed system-wide.

This option will make buildrepo continue build even if some package
fails. It is an 'build what you can' option.

The ABUILD_BLACKLIST can be used to give abuild a list of packages that
is known to fail to build. This can be used to reduce build retries
when building many/all packages recursively.

fi the signing key is missing we will fail. Rather than fail after
compile we check existance of signing keys before we start.

fixes #2270

This is supposed to be used in abuild only to make it possible to exit
with error early, before package is built, in case the signing key is
missing.

specifically, every script that reads/parses ./APKBUILD needs to set
CARCH.

Otherwise we might end up purging more packages than expected if an
APKBUILD has syntax errors

Some packages like gcc-gnat needs bootstrapping. We need to allow
circular makedepends so we can specify gcc-gnat as makedepends for gcc.

fixes bug introduced with c6b0de32 (abuild: add makedepends_build and
makedepends_host for crosscompile)

to detect if we cross compile