makefile: adapt firmware blob removal to changes for QEMU 8.2

makefile: adapt firmware blob removal to changes for QEMU 8.2

Namely, it's also necessary to remove .dts source files from the
meson.build file, because the .dtb file names are not directly listed
anymore since commit 6e0dc9d2a8 ("meson: compile bundled device
trees").

The same commit also introduced a "'.dtb'" in a line not just listing
a file name and removing that line would break the script. Be more
precise and require an alphanumeric character before the suffix.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>