Build fails while building oblfr because cross compiler is not in PATH

[fanginoort]

Building fails while building oblfr 2abd79917d67cc8213f71ee6da23d0ab83440f07 with error message

The CMAKE_C_COMPILER:
riscv64-unknown-elf-gcc
is not a full path and was not found in the PATH.

During investigation I figured out that the PATH is set to

output/host/bin
output/host/sbin

The compiler however is installed in

output/host/usr/bin/

edit:
I could fix it manually by adding usr/bin/ to the path by inserting a PATH=$(HOST_DIR)/usr/bin:$$PATH into boot/oblfr/oblfr.mk, however I think there should be a way nicer option to fix this.