Add build command to create an apk package from envkernel

Provides a quick way to incrementally compile a kernel and push it to device.

Example usage.

# Compile the kernel:  
$ cd /src/linux/  
$ source /src/pmbootstrap/helpers/envkernel.sh  
$ make tegra_postmarketos_defconfig  
$ make -jX  

# Package kernel and flash to device:  
$ pmbootstrap build --envkernel linux-samsung-p4wifi  
$ pmbootstrap flasher flash_kernel  

# Modify kernel source then incremental compile, package, and flash:  
$ make -jX  
$ pmbootstrap build --envkernel linux-samsung-p4wifi  
$ pmbootstrap flasher flash_kernel