raspberry-pi3: fix use of deprecated initramfs name (MR 2793)

initramfs-<flavor> is deprecated when using pmOS mkinitfs

This fixes #1253

[ci:skip-build] already built successfully in CI.

device/testing/device-raspberry-pi3/APKBUILD

 # Reference: <https://postmarketos.org/devicepkg>
 pkgname=device-raspberry-pi3
 pkgdesc="Raspberry Pi 3B/3B+"
-pkgver=0.1
-pkgrel=15
+pkgver=0.2
+pkgrel=0
 url="https://postmarketos.org"
 license="MIT"
 arch="aarch64"
@@ -36,7 +36,7 @@ weston() {
 
 sha512sums="
 1a735f34b5c8a6c1c860b84aef095ce675a287c2601405134f9f374fb6e88083265f9032655d3513ca83d0ddb6a169dadf8bceddfdec12a9c1bed3c9b67f8926  deviceinfo
-b95a66c8fa165aca2170ebb3f8688e78e1dd78d3ddb4d26c289ea77e406eec3bbfecb5a32997df86a692b674128d5b4f673c47f8e12013f33b765a69fec1cb51  config.txt
+ba195d9345fa4f4aee38cbc945b02f5bf13bd7e255124de006b4d04bc78c846c8c6b85268816f8a846d09a680e27d303006be89dd34de353f8cd8384c0fdbda3  config.txt
 a8e57535523d0f78e9d2ac1ac31b097a1a25468e3c910290647a60c862e631a4b4de94a5f6e4de999e1b9f70e67653a886967200856287dbc819b9f3ac16c548  usercfg.txt
 ae3de0b8fec07d3a283dc3c06bf8678eec1e65c9faf0b7f4fdc9fb92751e324d1f8e2fb224dbbf561b7e5a6fb34769bfa1657858375f74b101a130d78e0737e2  cmdline.txt
 7e5505cb07d5b4a81bd28443d508336b5c547356538f1c06f91ed93ad0d7d456d4f74f1d24df5a2e08c17e74f0a66607352ac4874e967e9a91dfec9522d2d58d  95-vchiq-permissions.rules

device/testing/device-raspberry-pi3/config.txt

@@ -3,6 +3,6 @@ boot_delay=0
 arm_control=0x200
 
 kernel=vmlinuz-rpi
-initramfs initramfs-rpi
+initramfs initramfs
 
 include usercfg.txt