grub: make backgrounds configurable in target.cfg

now under coreboot mainboards, target.cfg can specify
a background. if not specified, the 1280x800 one is
assumed, and used by default. it can be overridden.
the path should be relative to:
config/grub/background/

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/coreboot/t60_16mb_intelgpu/target.cfg

@@ -8,3 +8,4 @@ payload_memtest="y"
 grub_scan_disk="ahci"
 microcode_required="n"
 blobs_required="n"
+grub_background="background1024x768.png"

config/coreboot/t60_intelgpu/target.cfg

@@ -8,3 +8,4 @@ payload_memtest="y"
 grub_scan_disk="ahci"
 microcode_required="n"
 blobs_required="n"
+grub_background="background1024x768.png"

config/coreboot/x60/target.cfg

@@ -8,3 +8,4 @@ payload_memtest="y"
 grub_scan_disk="ahci"
 microcode_required="n"
 blobs_required="n"
+grub_background="background1024x768.png"

config/coreboot/x60_16mb/target.cfg

@@ -8,3 +8,4 @@ payload_memtest="y"
 grub_scan_disk="ahci"
 microcode_required="n"
 blobs_required="n"
+grub_background="background1024x768.png"

script/build/boot/roms_helper

@@ -60,6 +60,7 @@ cbdir=""
 cbfstool=""
 corebootrom=""
 seavgabiosrom="elf/seabios/default/libgfxinit/vgabios.bin"
+grub_background="background1280x800.png"
 
 initmode=""
 displaymode=""
@@ -362,13 +363,7 @@ build_grub_roms() {
 	# we only need insert background.png once, for each coreboot config:
 	if [ "${displaymode}" = "vesafb" ] || \
 	    [ "${displaymode}" = "corebootfb" ]; then
-		backgroundfile="background1280x800.png"
-		if [ "${board}" = "x60" ] \
-		    || [ "${board}" = "t60_intelgpu" ]; then
-			# TODO: don't hardcode this. do it in target.cfg
-			backgroundfile="background1024x768.png"
-		fi
-		backgroundfile="config/grub/background/${backgroundfile}"
+		backgroundfile="config/grub/background/${grub_background}"
 		"${cbfstool}" "${tmprompath}" add -f ${backgroundfile} \
 		    -n background.png -t raw || \
 		    err "build_grub_roms: cannot add background.png to tmprom"