Update toolchain location and detection

.travis.yml

@@ -40,9 +40,9 @@ install:
     fi
   - if [ $ARCH = aarch64 ]; then
     if [ $TRAVIS_OS_NAME = linux ]; then
-      wget https://developer.arm.com/-/media/Files/downloads/gnu-a/8.2-2018.11/gcc-arm-8.2-2018.11-x86_64-aarch64-elf.tar.xz?revision=90d1fe0a-42d7-439c-8ea6-bb7234b2f540&la=en -O aarch64-none-elf-linux-x64.tar.xz;
-      tar -xvf aarch64-none-elf-linux-x64.tar.xz;
-      export PATH=$PATH:$PWD/aarch64-none-elf/bin;
+      wget https://developer.arm.com/-/media/Files/downloads/gnu-a/8.2-2018.11/gcc-arm-8.2-2018.11-x86_64-aarch64-elf.tar.xz?revision=90d1fe0a-42d7-439c-8ea6-bb7234b2f540&la=en -O gcc-arm-8.2-2018.11-x86_64-aarch64-elf.tar.xz;
+      tar -xvf gcc-arm-8.2-2018.11-x86_64-aarch64-elf.tar.xz;
+      export PATH=$PATH:$PWD/gcc-arm-8.2-2018.11-x86_64-aarch64-elf/bin;
     elif [ $TRAVIS_OS_NAME = osx ]; then
       brew tap SergioBenitez/osxct;
       brew install aarch64-none-elf;

kernel/Makefile

@@ -141,6 +141,7 @@ else ifeq ($(arch), riscv64)
 prefix := riscv64-unknown-elf-
 else ifeq ($(arch), aarch64)
 prefix ?= aarch64-none-elf-
+hash $(prefix)ld 2>/dev/null || {prefix := aarch64-elf- }
 endif
 
 ld := $(prefix)ld
@@ -258,4 +259,4 @@ ifeq ($(board), k210)
 install: $(bin)
 	## baudrate no more than 600000
 	@python3 ../tools/k210/kflash.py $(bin) -b 600000
-endif
+endif