[RFC] Interface for installing bootloaders

[Jan Kiszka <jan.kiszka@siemens.com>]

Hi all,

for installing a bootloader into an image, a number of packages have to 
be installed into the buildchroot. These can be prebuilt packages or 
packages we build via Isar. Currently, we only handle the case of WIC 
requiring prebuilt Debian packages, and that in a hacky way (no proper 
distro abstraction, unneeded installation of unused dependencies). This 
proposal aims at providing a more holistic solution:

- Introduce new variables that express the two types of dependencies:
    - BOOTLOADER_PREBUILT
    - BOOTLOADER_CUSTOM

   So there is no "WIC" in these, any imaging recipe can use them.

- Imaging recipes (e.g. via classes/wic-img.bbclass) should
    - install all packages from BOOTLOADER_PREBUILT and BOOTLOADER_CUSTOM
      into the buildchroot
    - DEPEND on all recipes in BOOTLOADER_CUSTOM

- Define some common dependencies in conf/distro/${DISTRO}.conf:
    - GRUB_PREBUILT
    - GRUB_PREBUILT_append_${DISTRO_ARCH}
    - SYSLINUX_PREBUILT
    - ...

   Those variables can then be used to initialize BOOTLOADER_PREBUILT as
   needed and also replace those nasty BUILDCHROOT_PREINSTALL_WIC in
   buildchroot-target.bb.

- Set BOOTLOADER_PREBUILT or BOOTLOADER_CUSTOM in the
   conf/machine/${MACHINE}.conf or some overriding conf file - basically
   the same one that selects IMAGE_TYPE and WKS_FILE.

Anything I missed? Better suggestions? Eventually, when we can derive 
bitbake recipe dependencies automatically from package dependencies, 
PREBUILT and CUSTOM could be folded together. Right now, we need that split.

Jan

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