gnu: u-boot: Update to 2021.07.

gnu: u-boot: Update to 2021.07.

* gnu/packages/patches/u-boot-sifive-prevent-reloc-initrd-fdt.patch: New patch.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/bootloaders.scm (%u-boot-sifive-prevent-relocating-initrd-fdt):
New variable.
(u-boot): Update to 2021.07.
[native-inputs]: Add python-pycryptodomex.
(u-boot-2021.07): Delete variable.
(u-boot-tools)[arguments]: Adapt 'patch phase, disable failing test_spl test.
(u-boot-sifive-unmatched): Use default u-boot package.
(u-boot-pinebook-pro-rk3328): Use default u-boot package.