This is my simple guide to modify power kernel v45 image and to change or remove u-boot from it.

It needs a linux pc (or you can use N900 itself) do some steps.
With some modifications steps can be adapted to windows systems too

How-to remove u-boot:

1. Download power kernel deb file from repository and extract fiasco image from it
2. Unpack Fiasco image
   flasher-3.5 -u -F zImage-2.6.28-maemo45.fiasco
3. Remove u-boot from kernel image (starting read at offset 0x40000 aka 512 blocks)
   dd if=zImage of=uImage_pow45_noUBoot skip=512
4. Remove uImage header from kernel file to obtain a flashable zImage kernel
   dd ibs=64 if=uImage_pow45_noUBoot of=zImage_pow45_noUBoot skip=1
5. Flash new kernel to device
   sudo flasher-3.5 -f -k zImage_pow45_noUBoot -R

How-to change u-boot with Matan's version:

1. Download power kernel deb file from repository and extract fiasco image from it
2. Download Matan's u-boot from repository (it is named u-boot-for-power-kernel) and extract file image from it
3. Unpack Fiasco image
   flasher-3.5 -u -F zImage-2.6.28-maemo45.fiasco
4. Remove u-boot from kernel image (starting read at offset 0x40000 aka 512 blocks)
   dd if=zImage of=zImage_pow45_noUBoot skip=512
5. Copy u-boot to destination zImage
   cp u-boot.bin.0x38000 zImage_pow45_MatanUBoot
6. Add power kernel at end of previous file (starting write at offset 0x38000 aka 448 blocks)
   dd if=zImage_pow45_noUBoot of=zImage_pow45_MatanUBoot seek=448
7. Flash new kernel to device
   sudo flasher-3.5 -f -k zImage_pow45_MatanUBoot -R

ATTENTION: Actual Matan's u-boot can have problems with HW revision different from v2101

How-to add u-boot Matan's version:

1. Download power kernel deb file from repository and extract fiasco image from it
2. Download Matan's u-boot from repository (it is named u-boot-for-power-kernel) and extract file image from it
3. Unpack Fiasco image
   flasher-3.5 -u -F zImage-2.6.28.10-power48.fiasco
4. Transform zImage in uImage (necessary because u-boot boots Kernel only in uImage file format)
   mkimage -A arm -O linux -T kernel -C none -a 80008000 -e 80008000 -n "Power Kernel v48" -d zImage uImage
5. Copy u-boot to destination zImage
   cp u-boot.bin.0x38000 zImage_pow48_MatanUBoot
6. Add power kernel uImage at end of previous file (starting write at offset 0x38000 aka 448 blocks)
   dd if=uImage of=zImage_pow48_MatanUBoot seek=448
7. Flash new kernel to device
   sudo flasher-3.5 -f -k zImage_pow48_MatanUBoot -R

ATTENTION: Actual Matan's u-boot can have problems with HW revision different from v2101