Compiled AOSP image not booting and system partition is not mountable in recovery

I started porting Android 4.0.4 ICS AOSP to my device (HTC Desire SV) by creating a new devicetree from scratch.

I was able to compile and run the image for the emulator but the image with my devicetree isn't booting on my device (not even adb / bootanimation).
I just flashed the system.img with fastboot (first formatted it in cwm) and continued to use the old boot.img with my old kernel compiled from HTCDev.com which worked with the stock 4.0.4 ROM.

I am unable to mount the system partition in cwm after flashing the image:

Code:

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W:failed to mount /dev/block/mmcblk0p25 (Invalid argument)

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Formatting /system in recovery & make_ext4fs at the building steps:

Code:

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Formatting /system...
Done.
    Inodes per group: 7664
    Inode size: 256
    Journal blocks: 3831
    Label:
    Blocks: 245247
    Reserved block group size: 63
Created filesystem with 11/61312 inodes and 8007/245247 blocks
Done.

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Code:

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make_ext4fs -s -l 1004535296 -a system out/target/product/magnids/obj/PACKAGING/systemimage_intermediates/system.img out/target/product/magnids/system
Creating filesystem with parameters:
    Size: 1004531712
    Block size: 4096
    Blocks per group: 32768
    Inodes per group: 7664
    Inode size: 256
    Journal blocks: 3831
    Label:
    Blocks: 245247
    Block groups: 8
    Reserved block group size: 63
Created filesystem with 845/61312 inodes and 41780/245247 blocks
Install system fs image: out/target/product/magnids/system.img
out/target/product/magnids/system.img+ total size is 156374572

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After formatting in recovery I am able to mount /system again.

Partitions before flash:

Code:

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# cat /proc/emmc
dev:        size    erasesize name
mmcblk0p17: 00040000 00000200 "misc"
mmcblk0p21: 0087f400 00000200 "recovery"
mmcblk0p22: 00400000 00000200 "boot"
mmcblk0p25: 3bdffe00 00000200 "system"
mmcblk0p27: 0ffffe00 00000200 "cache"
mmcblk0p26: 4cfffe00 00000200 "userdata"
mmcblk0p28: 017ade00 00000200 "devlog"
mmcblk0p30: 00040000 00000200 "pdata"
mmcblk0p31: 46800000 00000200 "fat"
mmcblk0p29: 00011c00 00000200 "extra"

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Current BoardConfig.mk:
xttp://pastebin.com/VrdGpZrp (had to use pastebin my BoardConfig.mk content is somehow flagged by the board software)

I didn't yet include extraction of the blobs.
My plan was to be able to mount /system in recovery and to install the blobs with an update.zip until I got a booting system / adb logcat.

This is my first try at porting and I'm also pretty new to Android Roms etc.