Fix GRUB bootloader config dir: grub-pc → grub-efi

Build uses --bootloaders "grub-efi,syslinux" so live-build reads
config/bootloaders/grub-efi/ for the UEFI GRUB config. The directory
was incorrectly named grub-pc, causing live-build to ignore our custom
grub.cfg and generate a default one (missing toram, GSP-off entries).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/config/bootloaders/grub-efi/config.cfg

@@ -0,0 +1,31 @@
+set default=0
+set timeout=5
+
+if [ x$feature_default_font_path = xy ] ; then
+    font=unicode
+else
+    font=$prefix/unicode.pf2
+fi
+
+if loadfont $font ; then
+    set gfxmode=1920x1080,1280x1024,auto
+    set gfxpayload=keep
+    insmod efi_gop
+    insmod efi_uga
+    insmod video_bochs
+    insmod video_cirrus
+else
+    set gfxmode=auto
+    insmod all_video
+fi
+
+insmod serial
+serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1
+
+insmod gfxterm
+insmod png
+
+source /boot/grub/theme.cfg
+
+terminal_input console serial
+terminal_output gfxterm serial