pmb.aportgen.linux: cleanup makedepends (!1880)

Newer kernels require "bison" and "flex", so to easy porting we should
add them to generated APKBUILDs by default. While we are it, clean up
the default set of dependencies a bit:

  - installkernel is only needed when using "make zinstall", which
    we are not using in downstream_package (it requires special kernel
    configuration, so it's not really suitable for downstream...)

  - most kernels seem to compile fine without gmp-dev and elfutils-dev,
    so they should only be added when needed

  - the kernel has its own headers, why do we need linux-headers?
    (probably copied from Alpine, but anyway...)

  - add openssl-dev since it is needed for building/signing kernel modules

Useful reference: https://www.kernel.org/doc/html/latest/process/changes.html
Although it mentions much more than what is really needed...

pmb/aportgen/linux.py

@@ -10,7 +10,7 @@ def generate_apkbuild(args, pkgname, deviceinfo, patches):
     device = "-".join(pkgname.split("-")[1:])
     carch = pmb.parse.arch.alpine_to_kernel(deviceinfo["arch"])
 
-    makedepends = "perl sed installkernel bash gmp-dev bc linux-headers elfutils-dev devicepkg-dev"
+    makedepends = "bash bc bison devicepkg-dev flex openssl-dev perl"
 
     package = """
             downstreamkernel_package "$builddir" "$pkgdir" "$_carch" "$_flavor\""""