package/rpi-firmware: fix startup file names

package/rpi-firmware: fix startup file names

When booting, a Raspberry Pi will load the appropriate start files,
depending on the provided configuration. For example, if the config.txt
file contains ’gpu_mem=16’ the board will automatically load the
cut-down startup files (start_cd.elf and fixup_cd.dat on non-Rpi4).

Unfortunately, even when the appropriate version is selected in the
configuration menu, if the rpi-firmware makefile takes the good files,
it renames them to non-qualified, i.e. start.elf and fixup.dat. But as
these are not the files searched by the Raspberry Pi, the board will not
start.

This patch will set the names of the files to load as constant in the
config.txt file. This guarantees that the rpi firmware blobs do not take
any other corner-case decision based on any other as-yet unknown
conditions.

This eases the maintenance, as only the names of the source files
matter; the destination filenames are constants, and so are the
filenames in config.txt.

Fixes: #13026
Signed-off-by: Stéphane Veyret <sveyret@gmail.com>
[yann.morin.1998@free.fr:
  - very minor fix in commit title
  - drop the non-conditional macro and move its content into
    RPI_FIRMWARE_INSTALL_IMAGES_CMDS
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>